First party, I'd guess:
Rex/Pyra/Mythra
Springman/Ribbon Girl
The champions from BOTW
but here's some really "out of left field" ideas here, based on third party support for the switch in the first year!
Shovel Knight
Rabbids
Skyrim Guy
Travis Touchdown
... maybe even Steve from Minecraft, cause kids lose their minds for minecraft
I know that "No more Heroes" isn't out yet, but they did invite the creator to their press conference!