I know this would be really hard to implement, but I think it would be really cool if we a had 2 vs 2 match mode. It would make for some intense tournaments.
There are two ways that I think this could be done:
Straight out 2 vs 2
All 4 players are at the "table" at the same time. Each team shares an HP bar.
Tag Team
The format is an ordinary 1vs1 format, but players swap out at the end of each turn, along with their decks and hands. There are two ways to implement this one:
Shared HP
Both players have one common HP bar. When there is 0 HP on either side or a Player has no cards left, they lose.
Seperate HP
Each player has an HP bar of his own. When one player loses, his teammate can choose to surrender or keep fighting.