I would be very selfish and create an awesome 2d adventure with amazing graphics (just because I love 2d adventures).
I would add some rpg elements into it and the world would be extensive with exploration (a bit like Metroid, I know I´m a copycat but whatever works) over big areas.
I think my adventure game would be co-up (like Secret of Mana) and possibly online, there are far too few games with co-up mode.