Gonna have to disgree that Shantae isn't big enough to become playable. She and Reimu are my most wanted and both have earned a spot in the playable roster. However, Shantae doesn't work at all for a Mii fighter, so she will be playable or nothing.
It's off topic but I'm curious about what do you think Shantae has that other candidates don't, besides a really creative move set potential. Hundreds of characters have that so it won't make any character stand out. DLC so far confirms that it's not very important, only Hero, Min Min and Steve had really creative movesets. Besides, creative movesets cost more and this is the only reason we didn't get Steve earlier. Shantae's transmations could be quite costly if they want to use it well.
In my opinion, to be a legitimate candidate as third party, you need to be first or close to best representative for at least one aspect of gaming.
For example :
Joker : I really have a hard time understanding this pick but to at least it was RPG of the year at TGA the previous year (2017)
Hero : biggest jrpg franchise not in the game yet
Banjo : biggest platformer not in the game, at least on Nintendo platforms. Even then, I believe only Crash would top Banjo in platformers.
Terry : one of the biggest and oldest fighting game franchise, there are a lot of other big fighting games, but it's easy to argue Fatal Fury/KOF is up there
Steve : best selling game of all time
Sephiroth : most iconic villain not in the game yet, only competition would be Eggman
Shantae :
2D Platformer ? I believe Shovel Night, Hollow Knight, Celeste and probably many others are more deserving. There are also older ones like Ninja Gaiden.
Indie ? Same as before, add Touhou and Cave Story to that.
Joker pick ? Unless I'm mistaken, Shantae didn't get any notable prize
Sales ? I don't recall Shantae's sales is anything special
So I don't know what Shantae brings to the table. Sorry if I sound harsh but I always look at this things in a pragmatic way because I believe Nintendo doesn't decide these things in any other way.