I just evolved Grimer to Muk and, in so doing, completed my North American Pokedex. (I scored Ditto on Tuesday.)
So, what's next? Wait for Gen 2? Look for better mons with higher CP and IV from the current roster? Jump on a plane to London or HK for Mr. Mime or Farfetch'd?
A LivingDex. What's a LivingDex? One of each and every type of Pok?mon. Not just in the Pok?dex, but legitimately everyone, as in Charmander, Charmeleon, and Charizard. So when it's all said and done, you will have one of each. Want extra challenges? Got ya there too. One of each and every type of Pok?mon with maxed out IV's. That done? Max 'em out. Full power, top CP for all of them. That should keep you busy for a while.
EDIT: Side note, as I play the regular games, I have 801 Pok?mon to collect for a LivingDex. Even harder still to work out IV's as there are 6, plus EV's, and those only come into play with competitive. Even harder still, with natures and abilities added in to a massive move pool and trying to create fluid competitive teams, especially if you use Hidden Power as it is fluid type attack. Hidden Power has a set type for each particular Pok?mon, even two of the same type could be different, as it is based off IV's.