It'd be pretty fun. There could also be a system ala Final Fantasy 8 where you can convert monsters into cards as well. Players playing the game would have their characters on the ground sitting cross legged with some sort of small table on the ground. ( Just like how crafting apparatus magically appears) Spectators will be able to view a game in progress as well by pressing Home > Spectate. I wouldn't like games to be held in instances like market wards.

It would also be playable at tables or benches, that would bring a lot of more usefulness to those empty chairs you see at the adventurers' guilds and benches all around each city. Just like Final Fantasy 8 and 9, it could be part of the lore as well - a lot of people in their respective worlds would be able to play cards.