Veras Gunn
Yeah, I don't see this happening. Yes, NamcoBandai is developing it, but ultimately Nintendo has the last say on who is or isn't in the game, and random creatures from either Digimon or Tamagotchi probably isn't in their plans. I could see, at the very most, Pac-Man showing up in the game because he's such an iconic character and would likely fit in well with the likes of more cartoony characters like Kirby, Jigglypuff, or even Mario.
What about a Tamagotchi themed level? I know it isn't in the spirit of what Smash Bros. has always been, but it might could happen.
As for other iconic characters, I think that if Pikachu is in this, instead of that lame ball of sparks they used as his last final smash, this time it would be cool if he turned into Raichu and once her would used some big Thunder Storm type move. So like a vertical version of Brawl Ganon's final smash.