I stand by that Crash and Sans are the last two characters, with each representing something not in smash yet (No big western character heavily associated with a third party with crash seeming like the most popular pic for one, and Undertale is probably the indie game with the best chance to get in because of its popularity in Japan as well as the U.S. with a sequel in development).
People like Jill, Ninja Ryu, and Rayman don't have the pizzazz of being "OH MY GOD I CAN'T BELIEVE THEY ADDED THEM TO SMASH" that most of the dlc third party characters have had
Ryu - Most popular traditional fighting game character by a lot
Cloud - PlayStation Exclusive, had never been on a Nintendo console (except for KH Chain of Memories)
Bayonetta- From a niche third party series that is heavily associated with Nintendo. (Bayo is too violent!)
Joker - From a niche PlayStation exclusive series, that had very limited exposure on Nintendo platforms
Hero - From a mostly popular in Japan series that among the best selling series of all time in Japan, they even had school holidays when a new dragon quest came out for a while
Banjo - Owned by Microsoft, has been out of commission for 10 years and hasn't had a mainline platforming game since 2000 (not counting the gba one)
What do Jill and Ninja Ryu do?
Jill - Reps Resident evil, a fairly popular japanese series, but doesn't really break new ground. sure she's popular but usually the not crazy dlc characters that are expected at this point.
Ninja Ryu is just another classic NES rep like Megaman or Simon who is a lot less popular then both of them. There isn't a huge ninja gaiden fanbase nowadays unlike megaman or castlevania, so I don't really see a way that he gets in.
Crash represents another Rival to nintendo in the playstation 1 era who managed to get popular in Japan and America as an edgy alternative to mario, sort of like Sonic.
Sans represents Indie games and is here because he kind of represents the indie dream in a way, make a game heavily inspired by but different to a classic, get funded on kickstarter, and have legions of fans and critical praise across the world.
There aren't that many people that see Jill/Leon in smash and go out and buy a switch just to play as them, but there are a sizable amount that would do so for Sans and Crash.