Here is our current masterlist of deconfirmed characters: Dante, Master Chief, Cuphead, Travis Touchdown, 2B, and Ryu Hayabusa
Master Chief: Deconfirmed on Twitter by Frank O' Conner
Cuphead: Not happening, devs have talked about how they WANT to be in
Dante: "Get DmC on the Switch first", and that's only just
now happening. The director of the series was under that impression which means he hadn't spoken to Sakurai which means he isn't happening. (Thanks to Org. XIII for wording this better)
Ryu Hayabusa: An interview revealed that the team hadn't talked with Sakurai about Smash, but if they
did their first pick would be Kasumi anyway
2B: An interview near the game awards, the exec producer of Nier comments on how Nier wasn't chosen to be in Smash Bros but congratulated Atlus for Joker
Travis Touchdown: Deconfirmed by the creator himself
And some sources:
"Devil May Cry should be on the Switch first"
Interview deconfirming Nier, as cited in one of Papagenos latest videos
The only source I'm having trouble finding again was the interview about Kasumi and Smash Bros, but it's out there