For Mario characters, I think 4-5 could be acceptable. Princess Daisy, Bowser Jr., Geno (by popular demand, I just wish he had his own game first), King Boo (don't tempt me to make a moveset <<), and Paper Mario. For other characters, Princess Rosalina, imo, should def. be a costume. Mr. L could either be a costume for Luigi, or his Final Smash. One of my friends suggested that Princess Peach, Daisy, and Rosalina should be a switch-out triangle, like Zelda/Sheik or Red (Pokemon Trainer's actual name).
I had high hopes for Daisy being in Brawl, but so much for that. People may counter this in saying that Daisy will be another clone, but I beg to differ. On another forum, I made a moveset for her, completely different from Peach's. Besides, she deserves this spot more than Waluigi. At least she has background; Waluigi has none. I'd definitely be fine with Waluigi if he co-stars in a future WarioWare game, but I don't see him being in the next Smash over Daisy, imo.
Bowser Jr. is definitely a great character, at least to what I imagine him to be. There are so many possibilities for his Final Smash, therefore making Bowser Jr. a great overall character.
I'll be honest; I just finished Super Mario RPG a few months ago on an emu, but I loved it so much. Then I saw how Geno was overly requested in Smash, in coherence to Ridley. Geno would really make a great character, and not just that, it could seriously bring forgotten attention to Geno, hopefully persuading Nintendo to buy him back.
King Boo would be unique, but I definitely want him in. Ever since Mewtwo got removed, there haven't been any levitating characters. I want King Boo in for a unique way of playing, like Olimar.
Paper Mario was on my must-have list for Smash since I played and finished Paper Mario 64 back in 2000. They have two Links, why not two Marios?
This is just for Mario characters. I'll post and debate more Nintendo franchises later on.