I was just wondering if anyone has tried multiplayer variants with any success? I was just thinking that many of MTG variants might work as the mechanics are very similar. I was thinking 3-player cutthroat (Attack Right, Defend Left, abilities/globals anywhere) and Two Headed Giant might work well with this game, but haven't tried it yet.
I've played 3 and 4 player with the method you mentioned, just reversed. Attack left, defend right. Global & abilities against anyone. Also played 4 player 2v2 teams that way. Works well.
Also used one house rule. Global abilities like Phoenix/Mr. Fantastic's pay 1 energy to force a character to attack can be used to make your opponents attack out of turn. If I'm player 1, I can use this ability to make player 2 attack player 3. Possibly damaging #3, while opening up #2 for my own attack.