Waluigi-being shown as an assist trophy pretty much confirms his absence. I believe his assist trophy would have to be removed for him to make it.
Krystal-Star Fox Zero didn't really help the franchise and Krystal's lack of presence in recent years doesn't help either.
Impa-Her design in Breath of the Wild doesn't match what she was in previous titles such as Skyward Sword where she was prominent and relevant to the story. The assumption she would play like Sheik (possibly a Sheik echo) helps her case but Sheik's complete absence in the Zelda series in recent years doesn't help (and I don't count Sheikah armor).
Ken/Chun Li/MegaMan X/Zero/Protoman-Would need licensing from Capcom. X and Proto as MegaMan echoes I don't think would work.
Sephiroth/Tifa-Not relevant to current FF stuff and would need Square licensing.
Monster Hunter-Capcom licensing, lack of definitive character design and weapon choice. They could just use a bone GS and Rathalos armor and do what Inklings or Villager or Bowser Jr. did. But Monster Hunter gear was Mii DLC in sm4sh so that's most likely where we'd see anything MH again.
Dark Samus-Metroid Prime 4 isn't out yet to raise hype. The Federation Force & Metroid 2 remake on 3DS IMO are not the greatest to get people hyped. Other M was the last console metroid game and it was both hot garbage and several years ago. Plus with Ridley it might be hard to add another Metroid villain in, even with the potential to just make her a Samus echo.
Batman-Licensing and licensing.
Samurai Goroh/other F-Zero characters-It's been a LONG time since an F-Zero game was released. The series lacks popularity.
Eevee - Well ignoring the fact he/she is in a Pokeball again I guess it would be a lot of development time to create a viable moveset for him/her in which he/she would use different forms like Vaporeon, Flareon, Jolteon, etc. That's the only way he/she would be interesting also IMO. Eevee has always been relevant though being in so many Pokemon games and now with "Pokemon: Let's Go Eevee" being a thing.
This covers Eevee pretty well. I too was hopeful that Eevee's evolutions could be used as alts for the character but it feels more likely they will go with an evolution of one of the newer starters.
Raiden(MGS)-I'm surprised Konami was agreeable to let Snake back. Raiden isn't relevant in MGS5.
Quiet-I think a 2nd MGS character would be sweet and I do think she has a better shot than Raiden but I'd be shocked to see anything else from the series with the licensing needed from Konami.
Master Chief-Again licensing. Microsoft would likely never allow it.
Lara Croft-Yet again licensing. She also isn't much of a "fighting game character." Even though other characters have translated well to a platform fighter coming up with a good moveset for her would be tough.
Knuckles/Tails-If we can't get a non-blue Sonic my hopes aren't that high.
Pretty much with the Brawl additions of 3rd party characters like Sonic it opened up the potential for other characters. Sm4sh added some fan favorites like Ryu, MegaMan, & Pac-Man, furthering the potential of 3rd party characters. While a lot of popular characters seem like they could easily fit in smash pretty well the devs would have to work with more restrictions and the 3rd parties would have to be agreeable.
A lot of characters I personally really want in might have been super popular a decade ago but there's often a more recent video game of the same series that adds or focuses more on other characters making the characters I like (i.e. Krystal or Raiden) not really that relevant. Younger players would likely not be familiar with those characters as well.