I thought an alien was already pushing it... but D/P pushed it even further; pokemon capable of shifting time and space? Creation pokemon?
There can certainly be plenty more pokemon, but they're kind of running out of species of animals to base their designs off... and unlike Digimon, original designs that don't resemble an animal already in existence isn't their goal (it would make their job a lot easier however)
G/S/C: FRLG had exclusive pokemon (such as the legends and starters) however, GSC would offer no new pokemon to the series... a remake would certainly seem tempting (as it remains the best generation thus far) but they'd almost need a redesign... or another time skip, to keep us interested.
All in all, it's safe to assume a new pokemon title won't be coming out for another two years... not until they figure out their direction.