Honestly I would've said the Kirby franchise, considering the Mario, Pokemon, Legend of Zelda, and Kirby franchises seem to be the "Key 4" of Smash Bros. (3 biggest selling + Sakurai's own). However it seems that's already been fulfilled with Meta Knight and King Dedede. I think that's really the perfect rep for the franchise now.
As for other franchises, probably Metroid could get one more (not a fan but meh) although from what I've read it seems like slim pickings besides Ridley (Don't hurt me, Dark Samus fanatics
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). Even though you've suggested Donkey Kong Country I actually think Diddy was all they really needed. Star Fox yes, Krystal could fit to level out the franchise to atleast 2-3.
One last one I'd like to make mention of, although it's not essentially a franchise, is I think we need another 1 or 2 retro characters. It's just nice to see those 1-off blasts from the past characters, who don't really need anything else from their franchises.