Hmm...let's see....
1) A "logical" roster selected entirely on 1) Icon status in pop culture and 2) representation of Nintendo's series, projects and history is more important to me than a mechanically or aesthetically "diverse", "interesting" roster. (Note that I don't mean its more important than good mechanics themselves; its still most important that the game "feel good" to play. Just that I don't think characters should be included primarily because they'd be interesting to play or fill a conceptual niche.)
2) I prefer breadth of representation to depth. Mario, Luigi, Peach and Bowser should all be in because they're mainstream pop icons. But most series could probably just be represented by their main character. (Of course, I also want 0 cuts, so I guess we're well past this point anyway.) I would have preferred Chibi-Robo (or Andy or Ray Mk III or Isaac to, say, Rosalina.
3) I kind of think older secondary characters should trump newer secondary characters. I could be wrong, but I just don't think Nintendo has the mainstream pull they used to. So even though Rosalina might be the superior choice in many ways, I would have preferred Toad, just because he comes from a time when many more households were intimately familiar with what was going on in Mario Land. The same goes for PT's Squirtle and Ivysaur vs. Greninja.
On the other hand, maybe its important for Nintendo to behave as though they can still create cultural icons, whether its true or not, so maybe promoting new characters like Rosalina and Greninja really is for the best.
4) The perfect Smash Brothers game would have 0 cuts from the series overall. The only possible exceptions are Dr. Mario and Young Link, because they have such close spiritual successors in Mario and Toon Link, I'm not sure they really count as "cuts". (At this point, this kind of contradicts #1-3, but oh well.) Actually, I guess disliking cuts so much is why I'm a little skeptical about adding newly minted characters and secondary characters from well represented series--I'm wondering how much sense they'll make in Smash Brothers 7.
5) I prefer Roy to Marth. (Of course, I only know them from Smash, never having played FE. I get the impression my opinion would be different if I had.)
6) I want Pichu back (although I have 0 faith in the possibility). If they tried to actually make him a competitive character, with a "suicide rush" strategy, I might very well main him.
7) Despite having picked on Rosalina (and Greninja) a fair amount above, I find the character additions in Sm4sh way more exciting than the character additions in Brawl. (Of course, Pokemon Trainer was probably my favorite addition in Brawl, so its not all good news...) In particular, I think Villager and WFT are brilliant choices that represent the "new gaming" with a focus on non-violent, "life-style" games. (This is the same reason I'd like to see Mii, Chibi-Robo and, if possible, Professor Layton in smash. A lot of the most popular games now aren't about hitting things, and that should be represented...even in a game about hitting things...)
Wow...ask for my opinion and I'll just go on and on...