I don't know why so many people mentioned FE:Three Houses. If Rex didn't make it because the game released too late and we can assume planning for DLC has already been done, then no way Three Houses is included already.
Smash includes mostly japanese characters. No correction, they're all japanese characters. This seems odd, maybe Sakurai only talks to reps from japanese companies. But if he decides to include western gaming icons, two come especially to the mind and can't realistically be excluded:
Both are classic and well known gaming icons and the owning companies are currently on very good terms with Nintendo. I would also add Doom Guy, but I don't really see him as an icon - he doesn't even has a name. Duke Nukem would be different, but I don't even know who owns the IP by now and his franchise crashed and burned in the last period of time.
For the japanese 3rd-parties, most of the companies that are on good terms with Nintendo have at least two representatives: Sega has Sonic and Bayonetta (yes, it's still a Sega property), Konami has Solid Snake and Simon (+Richter), Capcom has MegaMan and Ryu (+Ken). But Square only has Cloud, and Namco (the company working on Smash) has only Pac-Man. So maybe these two companies get another rep? For Square the above mentioned Lara Croft would be a rep, but also the Slime from Dragon Quest. Slime has a Spirit, no idea if that is deconfirmation. For Namco it could be DigDug, galaga, Xevious, Tekken or SoulCalibur.
Another japanese company would be Level-5, which has with Professor Layton and Yokai Watch two franchises that would fit very well into Smash.
I doubt any other 3rd-party has a shot. Certainly no indies, I'm actually surprised Shovel Knight got a support trophy. Spirits are something else, and I would like Sans from Undertale as a Spirit.
For Nintendo we are scraping at the bottom of the barrel already. But yes, Elma from XCX seems certainly like a glaring omission, and therefore I think she is DLC. Captain Toad seems also like a possibility, as is Toadette. Besides that I can't think of anyone standing out enough to justify 6€