I like the idea of picking the pokemon first. However, I hate the idea of having everyone submit one pokemon and having to choose from hundreds of submissions. We need to narrow the field. Obviously, a concept or type could narrow the field. But, I think that is fraught with problems, for reasons mentioned by others.
I'd like to propose an alternative that hasn't been mentioned or explored yet. But first a little background reasoning....
One of the big problems I have with an evolution project in general, is it is a magnet for fanboys. Even more than the normal CAP projects, which are very fanboy-ish anyway.
From a competitive standpoint, there's really no reason to evolve a pokemon. If we want to satisfy a need in the metagame, creating a new pokemon offers more flexibility and creativity. The only reason we are even discussing evolutions is because the fanboy in all of us just HATES that
<crappy pokemon of choice> is a crappy pokemon. That's why I've always considered evos to be a bit contrary to the purpose of the CAP project. Because the point of the CAP project is to focus on COMPETITIVE pokemon play.
But just because evo projects have almost purely fanboy beginnings, does not mean they will not have competitive impact. The end result of evo projects will almost certainly affect the competitive metagame -- assuming that the new evolutions are competitively viable. That's the reason I am in favor of doing evo projects.
So, the dilemma is -- How to start the project?
I think we should start the project by deemphasizing the fanboy aspects, and instead focus on the competitive side of it. If we know we want the end result pokemon to be OU, perhaps we should first narrow down what tier of pokemon we will start with? From a purely competitive standpoint, I can envision three general categories of evolution, based on tiers (using some DP evos for examples):
1. BL to OU
"Power Boost"
Examples:
-- Rhydon -> Rhyperior
-- Porygon2 -> PorygonZ
2. UU to OU
"Major Upgrade"
Examples:
-- Gligar -> Gliscor
-- Sneasel -> Weavile
-- Yanma -> Yanmega
3. NU to OU
"Massive Overhaul"
Examples:
-- Togetic -> Togekiss
-- Roselia -> Roserade
-- Aipom -> Ambipom
If we used the starting tier as an initial voting topic, we could narrow down which pokemon are candidates to evolve. After that, you could take submissions or ranked voting to finalize the pokemon to evolve.
It's not a perfect system, but it is very clear for everyone to follow, since tiers are well-known. It also kicks the project off on a competitive note, which might encourage the project to follow a more orderly process. Because, I for one, fear that an evo project will be a big fanboy mess.