I'm trying to create a card that replaces an event happening for a single player with the event hapening for each player (specifically drawing and discarding cards). The wording I though of is something along the lines of "If a player would do X, each player does X instead," but I'm worried that it would continually replace itself and create an infinite loop.
Rule 614.5 seem to state that this is not the case, but since to my knowledge no actual magic card has a similar replacement I'm really not sure.
but I'm worried that it would continually replace itself and create an infinite loop.
Replacement effects can't create infinite loops, but they can create messy effects in multiples. Let's take the example of "If an opponent would draw a card, each player draws a card instead." in a game with 4 players.
If you have this and an opponent would draw a card, each player draws a card. If you have two of these and an opponent would draw a card, you'll draw four cards and everyone else will draw three. If you and an opponent each have one of these and another player would draw a card, starting with the active player, whoever has the later turn in turn order draws four cards and everyone else will draw three. If you and an opponent each have one of these and one of you two would draw a card, the person who tried to draw a card would draw four cards and everyone else would draw three.
And that's not getting into what would happen if there are three of them out there...
Shouldn't the second replacement effect replace each player's card draw from the first one with another "each player draws," causing the end result to be that each player draws four cards each? I'm don't understand how the card draw becomes assymetrical or how whoever draws the first card matters. Not that multiples will come up very often as the intention is for the ability to appear on a legendary creature, but still worth knowing if the eventual rules hassle is too much.
The first replacement effect changes "Opponent 1 draws a card" to "You draw a card, Opponent 1, 2, and 3 draw a card.", then the second changes all three "Opponent N draws a card" to "You, Opponent 1, 2, and 3 draw a card.", resulting in four "You draw a card"s and three "Opponent N draws a card"
The "who draws first matters" is because the order the replacement effects apply will depend on when they can apply or in APNAP order depending on how the draws happen.
This is an example of a mechanic that will work within the rules but hasn't been done because there are a lot of potential issues.
That would be true if the replacement effect only applied to your opponents' draws. The effect I intended is completely symmetrical as it replaces your draws as well, hence why the number of draws shouldn't be different nor would who draws first matter.
Wrong, at least if multiple players are low enough they would all deck as a result and leave nobody in the game.
This is a state-based action (704.5b), and thus will check every chance it gets; if the players do not draw simultaneously, the one who draws last would win in such a situation and if they do all draw simultaneously, the game is drawn.