Nice thread...it is a very difficult choice, and I think that helps me appreciate how good the roster is looking (even though none of my most most wanted characters are on it...yet). I voted for Miis, because I think they best represent what Nintendo has been up to for the past decade or so, but the fact that it was something like a five-way tie in my mind made me want to write out a "priority" list"
Essential Characters (Represent the Big developments since the last Smash was made; very close to being a 3-way tie for first)
1. Mii
2. Wii Fit Trainer
3. Villager
Vital Characters (Filling in some gaps; very, very, very close second)
4. Little Mac
5. Megaman
Brilliant Characters (Would miss dearly if cut, but, strictly speaking, doesn't seem "necessary")
6. Pac-Man
...(Honestly, part of me would like to see the last three cut, but I know a lot of people are excited for them so that's just mean and pretend I didn't type it).
7. Rosalina
8. Greninja
9. Palutena
Kind of a toss up on whether to prioritize Rosalina or Greninja. Is it more important to add new Pokemon characters, because Pokemon adds new characters with each (major) release, or is it more important to add new Mario characters, because they come along less frequently, and thus each individual one is more likely to have long-term importance?