X-Com
I think you have most of the parts you need for it in D20 modern already.
I think a RWBY RPG could be a really fun. I wonder how you could do that stylistic combat. Then I think you could do a lot of neat things with the weapons and even the setting.
A borderlands RPG could be really neat from a mechanics view point. You could have a selection of charts to just let you roll up random weapons for the players and the enemies. The whole thing could get crazy fast.