Okay, here's a current list of guesses of stuff, using seemingly credible leaks, echo guesses yada yada
ECHOES: (People bring up the Octolings but would they have a problem in having different ink colour than the Inkling alts?)
-Dixie Kong With K. Rool getting the newcomer treatment and echos being a thing, it'd be kinda weird if Dixie didn't finally get her spot in the spotlight after coming close to in Brawl's development. I'm not sure what the exact rules for an echo are if any, but could you just have her like Diddy, using DK's up special with her hair and maybe some hair usage in other attacks?
-Shadow the Hedgehog/Tails Miles Prower??? Tails and Knuckles are two way more iconic and important characters than Shadow, and got the Mii costume treatment previously. Having given the care to make Knux an assist, maybe they also did something to Tails like make him a semi clone of Sonic? (not sure if echos can include semi clones). Shadow's easier and more appropriate for an echo, but it kinda feels strange to have like, not even one of Sonic's MOST important characters (Eggman, Tails, Knuckles) represented and instead be given Ow the 'Edgehog. Sure he's notably requested but could you say the same about Tails?
-Megaman and/or Palutena echos that use custom moves possibly abandoned That could be Medusa and, uh, whatever Megaman character that isn't already deconfirmed. Could Roll fit the part?
-Isabelle I'm guessing if she's in, they'd rather stick to at least a somewhat unique echo fighter rather than a new moveset around, uh, building or something?
-Ninten Why not rep all three Mother games?
-Captain Falcon's echo It's time CF got a clone for someone within his series!
-Funky Kong Hey with Dix then you'd have a lot of the current DK crew in other than Cranky, who's chilling in that Melee stage.
-Ken/Ms. Pac-Man/Cloud's/Bayonetta's Why not?
-Some retro-looking Mario Could you imagine?
-Olimar's/Little Mac's/Fox's/Sheik's/Toon Zelda Blah blah fill up the character representation
Crazy NEWCOMERS guesses:
-Shantae Based on her Smash Ballot support (which ended up giving Indie Shovel Knight an assist trophy) and that slightly suspicious Pajamas Mode in one of her games
-Rayman If a couple of leaks are right and they're gonna be showing more Ubisoft/Nintendo crossovers with this
-Banjo Kazooie Vergeben's Minecraft thing plus open support from Phil Spencer in the Smash 4 era coupled with high fan demand
-Some retro Nintendo rep if that's not already covered by someone else Here's hoping for Bubbles from Clu Clu Land!
-High fan demanded character like Geno or Isaac If they went through the trouble of doing Ridley, Dark Samus AND King K. Rool despite his lack of revelancy, maybe they also did these two out of fan service despite lack of relevancy
-Skull Kid
-Gardevoir+Gothitelle echo+7th gen guy? A third generation rep's a third generation rep
-Chorus Kids/Any Rhythm Heaven rep Sakurai please!
-Heihachi Maybe Sakurai went back and tried to work with giving Heihachi a Smash moveset?
-Bandana Dee If he gets an original character slot but noone from Rhythm Heaven does...
Premature DLC guesses:
-Any of the above that don't make it in
-Options that missed base roster development by being too new and uncertain a Nintendo property (Springman, Rex and Pyra, new Pokemon)
-Crash Bandicoot for having a successful revival a while after the project plan was finalised