This is ONLY speculation! Take it with a grain of salt, please. I'm not saying that any of these characters will be there. Masahiro Sakurai did say there weren't anymore characters being added to Smash after Bayonetta & Corrin, but, you never know. Smash Bros for the Switch could have new characters to go with our current cast & DLC characters: Mewtwo, Lucas, Roy, Ryu, Cloud, Corrin & Bayonetta.
Smash Bros 3DS & Wii U have 60+ Characters (or 58 not counting The Koopalings & Alph separate), but, since it's coming to the Nintendo Switch, here's a list of possiblities for Undertale characters/ trophies:
Frisk (Costumes: Fell!Frisk, Swap!Frisk, Fresh!Frisk, Dance!Frisk, Mafia!Frisk)
Asriel (Fell, Swap, Shift, Alter, Dance, Mafia, Fresh)
Chara (Same as Asriel/ Frisk)
Sans (Alt Characters: Blueberry, SansyFresh, Raspberry, Nightmare, Dream)
Undyne (Same as Sans)
Asgore (Same as Sans)
Flowey (Same as Sans)
Mettaton (All Versions of MTT: Napstaton, Undyne, Sans, Freshaton, Papyrus)
Papyrus (Same as Sans)
Here's an example for Undyne:
Undyne (Trophy)- The ruthless captain of the guard, Undyne, is one of Asgore's most invaluable Allies. She's a fish monster from the internet sensation, Undertale. Though, we wonder if something is a bit fishy about her crush on Alphys.
Undyne (Trophy, Undying)- The Heroine only appears when a player does the no mercy route. But, in Smash, if she grabs a Smash Ball, you're good as toast because Undyne will strike you down, so you're going to have to try harder than that to defeat her!
Her Attacks Include: Gaster Blaster Shot (Neutral Special), Bone attack (Side Special), Undyne the Undying (Final Smash), Green Attack (Down Special/ Counter), Blue Jump (Up Special)
About her Final Smash: Sure it'd be like Mega Evolution, Wario Man, Giga Bowser, Giga Mac, Super Sonic, Beast Ganon & any other Transformation Final Smash, but, she'd be as strong as she is in the fight. It's impossible in Smash Bros to defeat her &/ or Counter her attacks due to invincibility frames.
Stage(s): Undertale, Ruins, Snowdin, Waterfall, Mettaton EX's Stage