Gill is obviously the overall ballot winner.
K. Rool was released before that?
Yep, K. Rool was Wave 3 alongside Chrom, Lloyd, Samus, Flying Man and the Black Knight.
Overall though, I think consideration should be given to all those released during the ballot's run as opposed to just those released after it was closed. Though if iirc that covers all DLC Mii Costumes.
If we use the Mii costumes as indication of characters that may have been highly requested but passed on, then I suppose the following would be the likely contenders:
Takamaru - A character we know Sakurai has considered numerous times. Solid contender for the next retro character, since adding at least one per game since Melee has been a tradition of sorts.
Isabelle - A very popular, if not the most popular Animal Crossing character as things currently stand. No surprises there. I think it's plausible to think she may be upgraded from an Assist Trophy if they add another AC character.
Inkling - Included for promotional purposes, but at this point it's generally agreed that they're likely given how big of a success Splatoon was and continues to be.
King K. Rool - A character we know has been highly (or at least somewhat safely assume based on fan outcry) requested. Though where he stands on the development side of things is an unknown, but the fact they acknowledged him at all outside of the usual trophy was a bit surprising. Not going to hold my breath, but I hope it means something more.
Viridi - Very popular Kid Icarus character. Should we get a another rep, it wouldn't be surprising if it was.
Ashley - Very popular Wario Ware character. Same deal as Viridi and Isabelle.
Chrom and the Black Knight are also very popular characters, but I'd wager the next FE rep will be from a newer title as usual.
Third parties are tricky, and I personally don't think any of the third party Mii costumes are an indication of possible characters, with the exception of:
Geno - Very popular Mario character. We know he was considered at one point. (Back for Brawl iirc)
Llyod - Very popular Tales character.
and to a lesser extent Heihachi - Sakurai briefly considered him, but passed since Pac-Man was who he really wanted to include.
EDIT: I forgot Dunban, Flying Man and Toad.
Toad I feel is a 50/50. He's had a decent following throughout the years, and now many have warmed up to the idea if it comes in the form of Captain Toad. Or it could just be because Toad is a well known Mario character/species.
Dunban is a popular character, though like with FE, I'd wager a character from a newer Xenoblade title is the more likely candidate for another character.
Flying Man was more than likely thrown in for the fun of it. More speculation on my part is that they perhaps took the remnants of the idea for having a 3D model for it in Magicant and revived it as a costume.