I'd love to see a full coop (no "game master" required) dungeon crawl style game. (board or cards), that allows you to play campaigns and level your hero.
Descent already has a decent campaign but to be honest this is much more of a strategy wargame than fast paced dungeon crawl. (moreover either the GM wins in Bronze or dies in Gold)

Ideally it should bring that "Diablo" hack & slay feel to the table, expanded regularely like an LCG.
Hell I'd even buy "one shot adventure boosters" that contain a quest and loot