honestly i think d4rk above me already notes the most important pokémon that need (pr)evos.. though i disagree with Absol. but that's just me.
apart from those, i gotta note Tropius. I thought he could maybe use a prevolution.
and secondly - but not necessarily, i just thought it would be interesting.. maybe an evolution for Castform? 'cause i actually really liked the idea of a Weather-dependent pokémon, but the storyline surrounding it stopped as quickly as it started.. XD;