If the RPG and the Adventure Game made love... they would have beautiful babies Seriously though, the one genre has what the other is missing, usually (although I'm not big on RPGs, yet). Adventure games put emphasis on puzzles and narrative, but often seem to lack freedom of choice or movement. RPG's feature strong freedom of choice and usually span a diabolically vast world, etc. And hey it's the age of hybrids, so why not try matching these genres up? Or what trep said, yeah, that.
*has a sneaking suspicion that somewhere, in a murky basement, someone is developing such an idea as we speak..