Well, they stated, that all cards from Battlegrounds will be playable in Standard MW, but some cards from Standard MW will not be playable in Battlegrounds. That makes sense Wizard tower in 1*3 space is not funny
You're thinking of Academy.
Battlegrounds is the expansion that comes with the puzzle piece board tiles and Domination Mode, along with official rules for multiplayer. Pretty sure I heard that somewhere a while back when it was being first talked about. There will be new cards in Battlegrounds too I think, with things that deal with all Mages on a team (ex: Heal all friendly Mages 3 dice). Stuff like that.
Edit: Laddin beat me to it