Since Opossum made me realise I could just use the smilies to make a roster, I'll just throw in my 2 cents of a slightly realistic but also mostly biased roster.
(Ashley)
(Dixie)
(Anna)
(Bandana Dee)
(Krystal)
(Decidueye)
(Isabelle)
(Xenoblade newcomer)(Inklings)
(Takamaru)
(Some other 3rd Party Newcomer)
all assimilated back into their source characters and dealt with exactly like Smash 4: if there's time at the end, they get cloned.
Inklings, Ashley, Dixie, and Isabelle are all popular, relevant and I love them all, so they're in. I considered Nook to also be added due to AC being a hit on every console but 2 might have been stretching it.
Decidueye and Mimikyu are rather interchangeable in this roster, as they've both got great moveset potential and are super popular. Decidueye kinda won out thanks to more being able to be done with it's body type. Either could also be good DLC.
Anna's got fresh relevance from Heroes, and she's been in almost every game. FE also needs less one-and-done protags. The next FE protag is likely a good DLC choice.
Dee just feels right. I'm not a super fan of them but the spear and parasol can be pretty unique.
Krystal and Takamaru are both a strong maybe. I didn't add Lip cause shes unfortunately lower than a strong maybe.
Xenoblade is getting rather popular, but I haven't paid any attention to it to say whos a good newcomer, so it's either Elma, that new guy, or that new girl. Hopefully Elma.
An ARMS character, I'd say Ribbon Girl, would be likely for DLC if the game is a hit.
I'd like to think they'd do something between a port and a new game where they just spruce it up a bit and add a bunch of new fighters, as Smash 4 feels pretty solid so there isn't much need to build it up again.