I'd like to see a whole lot of old Pokemon in Black and White, but like others before me I really don't want to see the National Pokedex upgrade happening again.
I want there to be as wide a variety of Pokemon as possible, so that when you go through a route, you can find more than 2 or 3 different new Pokemon. If the Isshu Pokedex had over 300 Pokemon in it from the get go, there could be 10 or more Pokemon per route, and suddenly it'd be possible to have loads of different teams by the first Gym, whereas in every other Pokemon game everyone has the same team by the first Gym.
In RBY it was [starter] + Pidgey, Rattata, Butterfree/Beedrill.
In GSC it was [starter] + Sentret, Hoothoot, Ariados/Ledian.
In RSE it was [starter] + Poochyena, Zigzagoon, Beautifly/Dustox.
In DPPt it was [starter] + Bidoof, Starly, Kricketot, Shinx, Budew.
Imagine if the first 2 routes in the game contained 10 or even 15 different species... It would make that first gym so much more interesting!