which ones seem plausible to you?
Off the top of my head, besides the ones have already been playable (and Ryu):
Banjo-Kazooie - This duo is more Nintendo than most Nintendo characters, being from the same canon world as Mario and Donkey Kong. The Banjo-Kazooie series is synonymous with Nintendo, and the 3 N64 games in which he appeared were beyond critically and commercially successful. Mario 64 created the 3D platformer, but Banjo-Kazooie perfected it. Would 100% be in by now if the Rare/Microsoft transaction had not been made.
Rayman - One of the most iconic video game characters of all time, very comparable to Mega Man. Many of his games are considered among the greatest of all time. Has recently been tied quite strongly to Nintendo too, which is a plus. And his trophy is in Smash.
Bomberman - Huge gaming franchise that has had a release on every Nintendo platform except the current gen 3DS and Wii U. Even had a cross over with Wario. Too bad Konami have killed the franchise.
Tails - Sonic could easily justify another character, and Tails is a hugely iconic character. Essentially the Luigi of the Sonic series. Sonic and Tails are among the most iconic video game duos of all time along with Mario and Luigi, and DK and Diddy.
Leon Kennedy - Resident Evil is one of the biggest gaming franchises of all time, and one of Capcom's big 3. Leon's games are more strongly associated with Nintendo than the other main protagonists, as 2 and 4 were both on Nintendo platforms, whereas 1,3 and 5 were not (barring remakes and ports).
Spyro The Dragon - Along with Crash, Spyro was a direct rival to Mario, just like Sonic. Skylanders has elevated him beyond what he was before.
Crash Bandicoot - See Spyro, above.
There maybe 1 or 2 others that I'm forgetting. All of the above have significant enough gaming legacies, and ties to Nintendo. Not saying that some of these characters will ever be in Smash, but they all deserve it.