Keyword Challenge

{Spoiler}Ooooh, quite the entry! From the initial explanation alone this is really well-thought-out and has some great flavor. It's also really exciting with the sheer amount of cards feeling like some kind of set preview hahaha. I think it's a nice and consistent choice with a lot of design space in this mechanic, though I do fear that a mechanic where the difference is purely in a permanent set of stats may lead to situations in which the card is only ever played in one way depending on the value of the effects. Well, that is more of a matter of execution though, and the mechanically certainly seems pretty excellent other than that one concern.

Dorkly Recruit - Pretty strong early mana generator all things considered. It can either be a 2/2 or a 1/1 ward and gives you mana. So it has versatility and can add mana that while colorless I don't think is effectively very different from colored mana in most decks (mana rocks can provide you additional colors, but creatures tend to give the same color already used to cast them anyway and in that sense you're usually using colored mana to pay for any color costs), and is harder to pick off in the early game because of that ward.
Disgruntled Bartender - Looks like a pretty decent card, a nice showcase of some of the baseline types that could go with this mechanic.
Deft-Handed Defender - Now this one feels hero-y, though I must say it evidences why you would stick effects on the citizen half. After all, this way you have little to no reason to play this as a citizen.
Lingering Private-Eye - I love this card. It has this dynamic where it can either go all vigilante and try to get as much info as it can by brute force, or it can disguise and be more sneaky about it, having protections that maybe with some equipment or enchantments could get it get through unharmed.
Convenient Telegram Booth - Straightforward and simple card to help you switch out a vigilante for a citizen or vice-versa. I like it. I do hear other attempts at stuff like this (sacrificing permanent for an ordinarily instant/sorcery effect) hasn't gone down well in the past though I don't know the reason (stuff like Font of Fertility). Either way though, it's a fun card on the imagery alone if nothing else.
Vanishing Sneak - I suppose I would need an image to better picture what is happening with this card's flavor. Still, I like the repeatable if slightly costly effect to swap between hero and citizen, which I suppose makes sense for a repeatable effect. I would probably make it have a more conditional unblockable though, seeing as on top of the other benefits this card can be a 3/3 unblockable.
Mob's Verdict - I initially figured this was some mob boss type of card but I guess I was mistaken. Well, it's fitting in flavor and I think in balance as well. Gives you more reason to play your creature as citizens too, which can be good for those that don't really get great benefits from being so on their own.
Heroic Vitality - Makes sense. Simple card, but quite acceptable.
Will of the Masses - The flavor I feel is a bit similar to Mob's Verdict. Not the same thing for sure, but close enough that I probably wouldn't want to see these two cards in the same set. I am also not sure if this really works for the kind of build-around-me-rare that you seemed to be describing in the intro to these last two rares, but it does encourage you to play citizens again, and as far as balance goes, it's simple and well-designed.
The Astrogazer - Very strong. I like how it takes advantage of one mode, while enhancing the natural advantages of the other. Feels like a good central card for this mechanic.

{Spoiler}It's a decent entry to this contest, certainly innovative in that it takes advantage of the "you don't have to make it a creature" allowance. Unfortunately, I think the mechanic is a bit of a one-faced wordier version of modal lands and messing with lands is always tricky, so finding space for innovation with this one would be as well.

Sweltering Mangroove - Seems like a decent card, and while dual lands usually enter the battlefield tapped, since this one is more like the modal lands that condition isn't really necessary I guess.
Forbidden City - While it's a simple enough effect this card would probably be a nightmare if implemented. Not because it's a bad card, but it's such a consistency boost and able to work on just about every deck that it would probably be ripe for banning pretty soon if it didn't cycle out.
Seasonal Flood - The fact this card specifies only cards you own outside the battlefield have the mechanic makes for a confusing interaction since this ability causes it to "enter the battlefield as", so the effect explicitly happens during that transition of out of the battlefield and into the battlefield, meaning I'm not sure if this card even works as it's currently worded.

{Spoiler} This is a really neat mechanic and one which not only has lots of design space to explore, but also has some great potential when it comes to strategic plays. Do you want to sacrifice a creature to get this benefit or to make your effect harder to remove? Or perhaps you'd rather have something to block or attack with? When should you amass artifacts and when should you help your army? The one thing I would have suggested about this is maybe making it an alternate cost instead of additional cost, at least from your designs there seems to be no benefit to them entering the battlefield first, and indeed an alternative cost would've let you make some cost cheaper opening up more design space.

Self-testing Tinkerer - Simple yet deceptively powerful. We're in an age of artifact tokens after all, so picture all those foods pinging each turn. Probably would've raised the cost.
Protective Scientist - Really strong, but given it doesn't do anything until you activate the ability and how costly that ability is, I'd say this is quite balanced. Would definitely want this in a format with some form of mass-artifact-destruction though, cause otherwise it might become impossible to interfere with any of your artifacts at any point, which could shut down the game in an artifact-heavy format like what such a mechanic would likely imply.

{Spoiler}I definitely don't think that's how the ruling would go for an ability worded this way. I think this is a spell ability or a triggered ability with the trigger set to entering the battlefield. Either way though, it's a one-time effect in the way it's currently worded. So you definitely can't equip something to it later and have the power and toughness increases switch.

That matter aside, it's a neat mechanic that reminds me of Unleash in a sense, sacrificing one of the creature's functions for an upgrade though in this case it's a sidegrade... by the mechanic alone anyway. Either way, it definitely looks fun and up for some strategic choices.

Viashino Turfguard - The card seems fine but we definitely have a disagreement on how this mechanic works that would affect how this creature's ability works so I'll refrain from saying much about it.
Forisyan Tactitian - I like how this card interacts with gaining defender by turning its double strike into a way to inflict its damage on two creatures its blocking. It's a neat and clever design.
Stonebark Grovemother - Not sure if I like that middle ability. The last one is nice as it forces you to make a choice between mana tapping or attacking. Giving it defender simplifies the choice, though you still can't tap it if you want to block later, but if you cast instants or stuff with flash you might get the best of both worlds. However that middle ability I can't decide if it seems a bit much, or if it is fine because the creature has such a high toughness anyway. I think I would probably up the cost by one or two mana.

{Spoiler}This mechanic kind of falls outside of the contest rules since its adding rather than a choice between states. Its not so far as off as to be completely disqualified, but ye...

Skyknight Trainee - A good stable to demonstrate the ability, a bear that can be upgraded. Not much to say, good simple design.
Containment Blaster - I like how the last two ability interact with each other without necessitating one another, though not to much the first option of hexproof. Hexproof is a very non-interactive mechanic and it just feels kind of slapped into this card as if filling a slot. I do like the cost of discarding a card though, non-standard costs are fun. Probably would need playtesting to know about balance in this case though.
Chromatic Obelisk - A cool mana rock that almost feels phyrexian in how you pay life for access to mana. I think it's strong but still decently balanced.

{Spoiler}I'll give this win to ZephyrPhantom for a great mechanic idea that is really in the boundaries of the contest plus flavorful and with a lot of well-designed individual cards. The runner-up is emberfire17 who's mechanic has quite a lot of potential and creativity behind it.

emberfire17 - A nice simple drawback mechanic that's easy to turn into an advantage or limitation with the right setup. I think the two designs you posted demonstrate this well - I wonder if there could be a red draw spell that lets you draw cards but gives you melancholy until your next upkeep or something like that. Stiff competition especially in the quantity area but the two ideas you do present do nudge you towards interesting strategies and make me want to think of potential future deck ideas.

Idea - It's a bit wordy and hard to make creative designs out of it as you've said, but I think there is room to make an archetype's worth of cards for this in a set and you demonstrate this well enough with a variety of on-damage and removal effects. One thing I think would've helped was to include a design that was something like:

Generic White Removal
Instant
Melancholy - Tap target creature. If that creature has power 1 or less, exile it instead.

Like, Abandoned Purpose is a great common to start off with, but I think something like this would help demonstrate that there are ways to tack Melancholy onto effects that aren't always hard removal or damage. Or maybe something like this?

Sacrificed Deputy
Creature
Melancholy - , , Sacrifice ~: Tap target creature. If that creature has power 1 or less, exile it instead.
1/2

Tarugg feels like he's playing with fire design-wise with casting from exile, but we saw a similar limited form of casting from exile with Kaya and I suppose if anything would get away with it it would be with the prime reanimator colors, conditional looping, and high casting cost.

kefke - I think the biggest hurdle for me was thinking of this as a mechanic without instinctively looking for the word "sacrifice", but Convoke appears in all five colors so I guess this works too. That said I think from a Melvin perspective this would end up reflavored as a Jeskai mechanic like "Creative" that paid for a different color symbol instead (or involve sacrifice of some kind) if it was making some hypothetical jump from Design -> Development. I think it works fine for a cube or custom set, but in its current form it is a little bendy.

melono - A more defender-like take on making this a drawback mechanic, with a normally overstatted creature being the first common to boot. Something I feel like some of the other entries struggled with was to show me how flexible the mechanic could be compared to its competitors; you address that pretty well here by demonstrating a variety of ways various colors could use the effect. Something I've run into when making keywords that work like this is that it's hard to make it intuitively feel like both a removal-esque drawback and a drawback meant to limit your card's power as well - I think your choice of effect avoids this problem nicely enough by tying it to tapped/untapped state.

vizionarius - You might've shot yourself in the foot there with the comparison on a Vorthos level, admittedly, but I do think it's interesting we have Culmulative Upkeep and nothing like this - says a lot about WoTC's hesitancy to make blatant drawback keywords. I'm a little more iffy on if this version of the effect should be running around as both removal and a drawback because it can't be intuitively tied to being tapped or not - very small complaint, mind, but I think it's a noticeable difference and I find your creature designs a lot more convincing partially because of it. It's a solid idea but it has a few small cracks in it compared to the competition, overall.

Sanfonier of the Night - After all this tapping and killing of creatures this is a very fresh take on the mechanic to see. I'm not exactly sure why Teardrops lead to Scrying, but then again, I'm not sure why Blood loots either, so I'll give it a pass. Your legendaries make for interesting value engines and demonstrate the mechanic well enough but given the competition I would've liked to also have seen 1-2 lower rarity cards that demonstrate how you could mess with the scry 1 a little - like, as is, especially since the two cards are Vampires, this could've created Blood and I would've have noticed the difference.

moweda - It's a simple but surprisingly unique evasion effect, and flanking has been a thing before so I could see WoTC trying their hand at a new version for a future set. I'm a little wary of Sabertooth Panther's impact in a draft but I suppose that would need playtesting to be sure of how strong it is. That said I feel like "despair" or "weaken" would be a word WoTC would go with so that it could be a bit more flexible in flavor than explicitly having to be linked to sadness. I think this is another instance of a good idea having to put up with stiff competition as this idea historically does work as a keyword; it's just that it's also very creature-centric and thus it's hard to get creative with it beyond its intended use, combat.

dangerousdice - Morbid but repeatable and in smaller increments is an interesting idea. I'm not sure if I like the designs they're implemented on though - Soot Ridden Mourner's drawback seems like it will never really matter for the decks that want it (to shove in some damage and then die while swinging) and Demonbound Duelist feels like it should be RB or monoB with that general setup. The effects are simple but unlike the competition I feel like it's a little harder to get excited about them because outside of Funeral's Benediction I feel like I'm not really convinced of why I should make a deck around this version of Melancholy, be it en masse or even using a single Melancholy card.

melono wins for striking the best balance between making a plausible keyword and making interesting designs to support it, with emberfire17 as runner-up.