Jynx316's Card Questions

Library Halls ) The way you were considering doing it would have been impossible as writ, but there are ways to do essentially what you intend to do within the rules; see Knowledge Exploitation and Stolen Goods for examples.

If an Instant or Sorcery would be forced onto the battlefield, the game negates the effect that would force them there and keeps them in the zone they were being pulled from. Also, in all cases, variable costs default to 0. (This is why it's a waste of time to try and cheat in cards with a variable cost.)

Fate ) Your example card isn't Fate's best foot forward, so to speak. The ability is niche enough that it's unlikely, even in Commander (where commander damage as a wincon is a thing) that it'd be used.

No - I am saying that the X would be treated as 0 for both grabbing and casting the spell.

X only has a nonzero value on the stack, and only if hardcast.

Another for this possibility could be to simply remove the X all together, along with "without paying its mana cost". "Exile Library Halls: Search your library for an instant or sorcery card. You may cast that card. Then shuffle your library. (You must pay all costs for that card.)"

As for the fate mechanic, it seems super broken. Look at Arc-Slogger, and keep in mind that exiling cards from your library really isn't a cost until you're in danger of actually being completely milled out. Dredge is another comparable mechanic - obviously fate is nowhere near that level of broken, for several reasons, but it is a comparison. There are a few possible fixes - returning the card to your hand rather than the battlefield would go a long way towards it. Putting it on top of your library is another option. Another option - although this may be getting into the realm of overcomplicated - would be to exile the top X cards of your library, and only return the card if one of the exiled cards has some quality, like sharing a card type with the permanent in question.

1st: I don't think that working out the mechanics for Luck as you want it to be will be worth the pain and agony to figure out how to do (it is going to be very complicated).

History and possibilities:

There are a number of cards that allow an opponent or target opponent to make a choice. As a general rule most of these cards haven't been considered fun mechanics by most people, because you are giving a portion of your turn to another player so either the card is potent regardless of what the opponent chooses like Fact or Fiction or the choice your opponent chooses is reduces the effectiveness of the card beyond the casting cost such as Browbeat.  This is just like the cards that rely upon coin flips such as Mana Clash, there are a small select group of players that like such things but generally they are not considered fun by the majority of players.

I know of none that do so without giving the player knowledge of what they are bidding on.

BUT

There is a keyword Mechanic specifically designed to create mystery and that is morph.  The key to making morph work is that the other player has no notification as to what is face-down except the colors you are playing with in your deck.  If you wish Luck to work then you will need to follow suite on this as well.  You will need to make a standard casting cost for "Luck" spells, and then to vary the actual cost to vary based upon spell potency you would need some other way to activate the spell.

I don't know where to go from there but be aware that Morph is probably one of the most complicated mechanics that Wizards has come up with so replicating portions of it aren't for the faint of heart.

Personally if I really wanted to do what you are suggesting I wouldn't create a new mechanic, I would make a cycle of morph cards that include your opponent choosing Up or Down (or Left or Right, but keep the choice binary) when the morph effect is activated.  The card would have different stats or effects depending upon which your opponent chose. Especially if the cycle used X CMC effects this should achieve your desired result.

Can other people ask questions here, too?

I have a more general question about card design: In which color would you place cards that make a player skip a turn? This hasn't really been done before, afaik the only card so far is Stranglehold, and that isn't even really an example, because it only affects extra turns. I realize the mechanic is very similar to taking an extra turn in most cases, which would put it in almost by default - but I could also see it in or . What do you think?