Greetings and salutations, all ye denizens of the Crucible!
I'm starting this thread to gather together the various house rules and tweaks that others have invented for multiplayer KeyForge. It's useful to hear how other players have resolved strange card interactions and counter-intuitive situations in order to keep multiplayer games as fun and fair as possible, especially in this, the early dawn of the world's first unique deck game, before any official multiplayer rules have chiseled into what I presume is the towering, onyx Obelisk of the Law at the center of FFG headquarters.
To start things off, I'll give a lenghty report on how our group resolves a situation in which multiple "Interdimensional Graft" are played by different players. ("Play: If an opponent forges a key on their next turn, they must give you their remaining [Aember]"):
Say that player turn order is A,B,C.
Player A plays Interdimensional Graft. Player B also plays Interdimensional Graft. Player C forges a key with 1 Aember remaining.
Who gets the remaining 1 Aember, A or B?
To resolve this, we came up with the "Turn Order Effect Check." Essentially, when a condition is potentially met, i.e. an "IF this THEN do this" effect, we look for the first effect that cares about that condition in turn order, starting with the active player.
In this scenario, the next player in turn order is player A, and so the condition of player A's Interdimentional Graft is satisfied, and so player A gains the Aember. At this point, we consider the condition "a player forged a key" and "the first instance of Interdimensional Graft found in turn order" to have "cancelled each other out," and so the effect check stops.
Player B's Interdimensional Graft is not cancelled, however. It will simply hang out until all other players have take their turn, waiting to see if any other players forge a key.
For example, suppose the Aember gained by player A puts them at 7 Aember, and they forge a key at the beginning of their turn. Just as before, the "Turn Order Effect Check" proceeds in player order. Player B's Interdimensional Graft is still hanging out to see if its condition is met, and so player B gets player A's extra Aember.
So, that's how we handle this situation. It's not clear to us at this point if the "Turn Order Effect Check" is generalizable, but should work for effects similar to Interdimensional Graft, where attempting to resolve effects simultaneously leads to ambiguous results.
Anyhow, hope this was maybe helpful. Happy forging!