Well, every region has a couple of pokemon it needs for sure. Stuff every gen has had, like:
-the three starters
-a couple of bug pokemon for early routes (it sounds like Nino's idea of one caterpillar with many evolutions could fill this slot)
-a bird pokemon or two for early routes (think pidgey/spearow/starly)
-a rodent for early routes (rattata/zigzagoon/patrat)
-a trio of legendaries (like Articuno/Zapdos/Moltres or Raikou/Entei/Suicune)
-Main legendaries (looks like Solrock and Lunatone?)
-Event legendary (mew/celebi/shaymin)
These are just the ones I can think of off the top of my head. A lot of the rest of the dex is open to interpretation. Every gen has added between 130-150 new pokemon, so that's about how many we'll need. If it's alright with everyone, I'd like to create a blank image numbered 1-130 that everyone can update with new sprites or ideas?
EDIT: Think something like this. The boxes are for the actual sprites and there's enough room above them for people to add a name or quick description of their idea.