With regards to characters currently part of the roster who I want to return, I most want Ike to remain a playable character. He's my favorite character to use and he plays differently enough from Marth and Lucina that I want to see him stick around. Theoretically he could be at risk because, despite his popularity, his games (Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn) didn't sell well and he isn't as relevant from a business standpoint compared to characters from more recent releases. That could be remedied if they ever port those games onto the Switch or any future Nintendo console, but yeah. I could potentially, and unfortunately, see his playstyle being kept around while he himself is replaced with some new flavor of the month character but it would disappoint me since Path of Radiance is my favorite video game. I'm not going to get my hopes up about his future in Super Smash Bros. games, but I'll definitely be happy if he sticks around.
As far as new characters I'd like to see added are concerned, here are a few:
Shantae - Her games have been on Nintendo consoles since the first one was released on the Game Boy Color. The series has been branching out onto other platforms in recent years and has been gaining more attention and popularity. Her animal transformations would make for fun special moves, you can use her dancing as taunts, and there is some great music to use if they wanted to utilize that. If they pull it off well I think she'd be a fantastic addition to the roster.
Decidueye - I don't really think Decidueye has a very high chance of being added, but I really want a Grass representative so all three starter types of Pokemon have acknowledgment on the roster. I can see Decidueye being a fun character to play, but I struggle to see public perception of an archer being anything other than a rip-off of Link as far as Super Smash Bros. goes.
Bomberman - I've seen his name thrown around, the series has been around for a very long time and has a history on Nintendo consoles, and had a game come out for the Switch a year ago. The moveset should be easy to figure out and the alts are a no-brainer. I think it'd be a good choice to add him.
King K. Rool - Never been into Donkey Kong and I have no attachment or stake in his inclusion, I just see him requested a lot and would like to his fans to have their wish come true. We could use more villains anyway.
Isaac (from Golden Sun) - Another character that I have no attachment or stake in that I'd just like to see added to please the fans who have wanted him included for years.
As for who I'd like to see added that I know will most likely not be added:
Banjo & Kazooie - I very much doubt Microsoft would be up for it, but people really want them to be included.
The Black Knight - He was referenced in the Super Smash Bros. for Wii U and 3DS games through a DLC Mii Outfit. He would be another good villain character, but people complain about too many Fire Emblem characters as it is and he's just another sword user, albeit an interesting one with a neat look.
Duster - I would like to see a non-PSI character added from the EarthBound/Mother series and I think Duster would be a good, if not the best, choice due to his Thief Tools. His kicking moveset would also be interesting to see. He probably wouldn't be added though since he canonically has a limp in Mother 3 and I don't know how they would get that across in Super Smash Bros. It would kind of ruin the mystique too, so it's probably better that he not show up.
Hector/Ephraim/Any Axe or Lance User from Fire Emblem - They're not going to add Hector or Ephraim since their games are old and no longer relevant, so you really have to just cross your fingers that at some point down the line Intelligent Systems decides to make another Lord character that uses an Axe or Lance and that the release of their game coincides with the Super Smash Bros. release cycle. I highly doubt the Lord in the upcoming game for Switch will use an axe or lance. I'm not getting my hopes up, but I'd love for them to add an Axe or Lance character not only because I prefer the older Fire Emblem games, but to show casual players who only know FE characters through Smash Bros. that it's not all just anime swordsmen.
Simon Belmont/Other CastleVania Representative - I've seen Simon Belmont in particular suggested a few times. CastleVania, back in the day, was admittedly a big deal for Nintendo and I could see it happening but I moreso doubt it.
Kumatora - If we have to have another PSI-user from EarthBound/Mother, I think Kumatora would be a great choice. She can use other PSI moves that Ness and Lucas don't use and I imagine PK Ground would be a good choice for a Final Smash.
Lyn - She's one of the more popular Lords from the Fire Emblem series. She's already an Assist Trophy , so it's not outside the realm of possibility. I don't really know what sort of moveset they'd give her though outside of the obvious fast slashes, which doesn't seem particularly deep, and as such it would seem unlikely.
Waluigi - He has a dedicated fan base who loves him but Nintendo never bothers to do anything with him other than throw him into side series like Mario Party or the myriad Mario sports games. As such, he doesn't really have a lot to work with practically.
Aside from all that, there are some characters who I think might show up that I don't care about but figured I'd mention:
Crash Bandicoot - He's a popular character and the first three Crash Bandicoot games are coming to the Nintendo Switch. I think it's possible.
Whatever New Lord is in Fire Emblem Switch - Fire Emblem has gained a lot of popularity in recent years and ever since Roy was included in Melee, Intelligent Systems has utilized Super Smash Bros. as an advertisement for their current game. This one is guaranteed. I just hope the game, and the character, are worthwhile and interesting enough to justify it.
Rex and Pyra - Never played Xenoblade but it seems to be a rising star in the Nintendo realm and I've seen them suggested.
Shovel Knight - I never got into the game but he was highly requested during the Smash Ballot. He also has an amiibo, so I'd say it's possible if not likely.
Spring Man and/or Ribbon Girl - I've seen them suggested a few times and imagine they would be neat characters. I never played ARMS so I don't know if there's a better choice as far as characters go, so I went with the most obvious.
Viridi - I'm not a Kid Icarus fan, but Uprising seemed popular and Sakurai did work on that title. Having seen her suggested numerous times, and considering Sakurai's personal involvement, she seems plausible.
