I have to say that I only want newcomers that actually bring something new to the table and are interesting. That's something I really liked about the newcomers this time around: they're all interesting (except for Dark Pit and Lucina). Rosalina has Luma, Duck Hunt is a trio, Robin has his books, etc.
That being said, there's also something that can't be ignored, and it's how popular characters are. My choices would be:
Toad: I don't really like the idea of Toad being playable, but it's undeniable that he deserves the spot. He could be the smallest character (a few inches smaller than Olimar) and the third fastest after Sonic and Captain Falcon. I don't care about Peach's B move, as they can easily change that Toad or even have two Toads at the same time for all Sakurai cares.
Paper Mario: Mario having eight reps, LOL. Paper Mario could be a unique character with a very interesting moveset. He could even glide (given that feature comes back) by transforming into the paper plane we saw in Thousand-Year Door.
Dixie Kong: without a doubt the third most important character in the Donkey Kong universe. She could use her ponytail to float like Peach but with a little twist so both of them stay unique.
Cranky Kong: another iconic Donkey Kong character that would be hilarious to fight against. An old monkey that smacks people with a cane... Hahahaha.
King K. Rool: I'm not really sure about this one since he's fallen into irrelevancy lately. However, if he gets a role in a new Donkey Kong game he should really be in. I see him more as a boss, though...
Impa: the only Zelda character that's not already in who deserves a spot. She's been in pretty much every Zelda game and would play similary to Sheik but with several differences. Not even a semi-clone.
Krystal: once again, it all depends on how relevant she is by the time Smash 5 comes out. If she's featured in the upcoming Star Fox Wii U game, let her in. If she doesn't, they might as well leave her out.
Inkling: this is an obvious one for the next game.
New Pokémon: because we're sure to get the "it" Pokémon when the new Smash comes out.
Now the cuts. It's obvious I want Dr. Mario, Lucina and Dark Pit out. They should have stayed as alternate costumes as it was originally intended.