Besides the obvious D4 or D1/2 remakes I really would love to see Blizz make a FPP CO-OP dungeon crawler using what they've learned with the Overwatch engine.
Multiple classes, xmogs, lotsa loot, drop-in co-op. There really aren't many games in that genre, and if they make a AAA title it'd be king.
Some may say go play WoW, but that is a traditional 3rd person MMO with dungeon/raid mechanics/cooldowns. I'm talking more of a Heretic/Hexen/Skyrim vibe but co-op.