The Mario series has gotten at least one rep each game (Wario counts for his own series but originated in the Mario series unlike DK and is a mainstay in all the spin-offe), as has Pokémon, so there's two trends we can guarantee for SmashNX.
While we cannot speculate on Pokémon considering the new gen isn't revealed at all yet, the only possible character left for Mario has to be Toad, because he's considered an all-star and is one of the highest-profile characters Nintendo has who isn't playable yet, as well as being the only playable mainline Mario series character not playable and the only Mario Kart 64 character who hasn't yet made the transition. Just make Peach's counter his and give Peach a new special and it's not even a problem.
Inklings are, as someone mentioned before, the Olimar of SmashNX, and they're likely to have sequels and be a guaranteed lock.
Dixie is likely to be relevant for a long while but if K. Rool returns to his status as main villain in an upcoming DK game the both of them are most assuredly a lock.
Style Savvy is a super successful series with loads of popularity in Japan, and I can very easily see the Savvy Stylist as the WFT and/or Villager-type character for SmashNX.
Ice Climbers are definitely coming back, Wolf certainly has the popularity as well but may end up in a Mewtwo situation.
Takamaru is the default Retro rep with the likeliest chance considering his popularity as expressed through an Assist Trophy and now a Mii Costune.
Beyond that, we'll probably be getting a new FE character, Tingle or Impa for Zelda as the other mainstay characters, an XCX character, and I can't speculate on smaller franchises (besides Advance Wars, highly likely to be getting a new game and therefore a rep) or 3rd-Parties since we don't know who they'd be haha, though I wish Wonder Red had the popularity to justify his inclusion.