There are the echo fighters that I feel like at the most likely. I'm not going to include Street Fighter, Mega Man or Bayonetta echos as I don't know much about those series.
Likely
1. Octoling
Octolings have been popular for a long time and there wouldn't even need to be substantial model changes as they just have an eye texture and hairstyle difference. Splatoon has also been such a huge success of a series that I wouldn't be surprised if they wanted to add another fighter. Plus, Octoling DLC just released for Splatoon 2. The timing couldn't be more perfect.
2. Dark Samus
Samus had her Dark Samus costume replaced in the demo. That doesn't necessarily confirm Dark Samus, but it seems unlikely they'd remove it for no reason. And considering the hype behind the Metroid series as of late, I wouldn't be surprised.
Maybe
1. Funky Kong
The people love Funky Kong, he's recently become a meme and he'd be a great Donkey Kong echo. But K. Rool and Dixie Kong are already at the top of everyone's wishlist, does he have a chance as well?
2. A Fire Emblem Echo
There's a lot of candidates here, so I'm not going to point out a specific one. But there has been a negative reception to the sheer amount of Fire Emblem fighters added, so it raises the question of whether they'll add another Fire Emblem echo in spite of that.
3. Dixie Kong
More appropriate as semi-clone or a full on new fighter. Being an echo wouldn't do her justice. But it's possible.
4. Mecha Sonic or Classic Sonic
Sonic is a huge series, and it's due for another character. These would be two easy echoes that could take from Sonic.
5. Ms. Pac-Man
Add a bow to Pac-Man. Do small frame data adjustments. Call it a day. Bandai-Namco could easily make her happen and surprisingly she wasn't a costume for Pac-Man in Smash 4. There are a lot of echoes that I'd rather have before her, though.
Unlikely, but possible
1. Dry Bowser
A moderately popular Bowser clone found in most spinoffs, and a rather easy inclusion. Gotta get worried about Mario series rep oversaturation, though. Especially when we already have a Mario series echo (Daisy) and there is demand for other characters like Waluigi and Birdo.
2. Shadow
Despite his popularity, being an echo wouldn't do him justice. He's much more than a Sonic clone.
3. Isabelle
Popular, and could easily take from Villager's moveset. But I forsee her just staying as an assist trophy, especially considering that being an assistant is her job!
Very unlikely, but still possible
1. Birdo
Too different from Yoshi to be made an echo, although it's possible they might take some creative liberties and give her the ability to suck in enemies instead of grabbing them with her tongue.
2. Ninten
Has a small fanbase, but with the relative obscurity of Mother 1, it seems rather unlikely.
3. Viridi and Medusa
Kid Icarus likely isn't going to receive another rep for a while and these two characters are too dissimilar to be made just an echo.