If the newcomer spots are limited because of bringing everybody back, and literally everybody being in the case is a result of fanservice and fan demand, and with Inklings, Daisy and Ridley all being fan-favorite characters and heavily requested, then it stands to reason that all the newcomers, Echo Fighter or no, should theoretically skew towards heavily-requested fan-favorites as well.
Which bodes well for characters like K. Rool, Dixie, Geno and Isaac that Smash fans have been requesting for years alongside Ridley and Daisy, as well as Ashley, Isabelle and Skull Kid who are currently MIA on the Assist Trophy side. Especially to the latter, we've seen a lot of fan-favorites either reprise their appearance as AT's or show up as one (Waluigi, Lyn, and Takamaru / Krystal, Bomberman, and Knuckles, respectively), so the fact that we can't find those three anywhere as of yet seems to be suspiciously significant.
I would personally bet on K. Rool and Geno for sure, Ashley and Dixie as a lesser chance, Skull Kid as a darkhorse pick, and then Isaac and Isabelle's chances as non-zero but not all that likely IMO (though I'd hope to be wrong!). But you also take into consideration Simon being leaked (and c'mon guys, if you don't believe Simon you're setting yourself up to look like fools because he's about as confirmed as you can get with the leaks he was a part of being basically confirmed in everything they revealed except for him showing up and the Castlevania compilation, so it's a matter of when he'll be revealed, not if), rumors of Minecraft and/or Microsoft involvement (which could implicate Banjo as a non-zero possibility), as well as Elma being pseudo-confirmed with the Mecha figurine that Sakurai bought and tweeted about; and you've already got a pretty darn good newcomer line up.
And aside from Dixie, that's not even including potential Echo Fighters! Of which the way it was revealed heavily implicates that we're see more of than just Daisy. So we may get less newcomers in the base game, but in terms of who we and the fanbase at large have been looking forward to for years upon years, I think we'll be generally satisfied with who we do get.
And then you have the very likely possibility of post-launch DLC, which is a whole other ballpark in terms of characters, 1st- and 3rd-party, that could show - characters like Spring-Man (And Ribbon Girl as his Echo), Rex & Pyra, Crash, and Spyro.
This is really going to ultimately hehe be the ultimate roster and Smash game in general by the end of its lifecycle.