When discussing Battlegrounds, it would be nice to be able to link to the Battlegrounds versions of cards instead of the regular Hearthstone versions, such that it includes the tier rating instead of mana cost.
I'm not sure what the syntax should be. One option would be to have a separate tag like {bgcard} instead of {card}. A different option would be to have a prefix in the body, such as {card}bg:Alleycat{/card}. I guess the cleanest solution is an attribute, such as {card set="battlegrounds"}Alleycat{/card}, but that would be terrible to type.
A different approach would be to have the post editor look up the card as you're typing and have a way of cycling between versions of the card. Misspelled card names are the #1 reason for me editing my posts, so a way to verify that the right card will be displayed would be great. But this is probably the most work to implement.
With Out of Cards supporting multiple card games, I guess you'll eventually run into overlapping names between games as well. Especially really generic names such as "fireball" might exist in multiple games. So it might be worth thinking of a solution for disambiguation in general and not just in the scope of Battlegrounds.