Someone source where Sakurai said he doesn't want to add many characters? It sounds like an extreme misconception or mistranslation. He's said "I don't simply want to add a bunch of characters and a bunch of stages" but he was talking about having his series evolve, not just stuffing it with characters with every installment.
King K. Rool: Extremely Popular
Dixie Kong: Decently Popular (Moreso in Japan), and was planned for Brawl at one point (So we know Sakurai approves of her)
These are the pros to the two characters, it's up to you to predict who you think will be in Splash.