The reason I brought up characters like Crash, Spyro and Rayman was because most of gamings icons are already in Smash, There isn't much star power left out there, The way I see it we have to go one step below the "Icons" and there is different tiers/levels for character's.
Icons/Legends: (The biggest names in gaming) Mario, Donkey Kong, Pikachu, Sonic, Pac-Man, Mega Man and Snake ,etc.
All-Stars: (Not as iconic or legendary, But were still popular and successful franchises that had an impact) Banjo-Kazooie, Crash Bandicoot, Spyro The Dragon and Rayman.
Crash Bandicoot (And to a lesser extent Spyro) while associated with Sony more were actually mascots for the Playstation brand, Not to mention Crash was Mario's rival in the late 90's and was a very successful franchise and both got remastered trilogy's for the Switch Plus a lot of people (Myself included) would love to see Mario and Crash go toe to toe.
Rayman while not as big is Ubisoft's mascot and has gotten some pretty big 2d platformers over the decade for Wii, Wii U and Switch not to mention Nintendo has started having a strong relationship with Ubisoft and developed Mario Kingdom Battle/ Donkey Kong Adventure and Starlink/Star Fox and Rayman even had a trophy in Smash Wii U.
I'm sorry if I sound like I'm coming off harsh ,But I'm just telling it like it is, Shantae was never a big name character to begin with and isn't even in the same league as those 3 up there, Let alone with the biggest names in gaming and while she has had a few games most of them were just Eshop/PSN/Xbox Live digital releases that while well received and do well for the indie scene, They were never big releases or huge sellers. Your average gamer is likely gonna recognize Crash, Spyro and Rayman a lot more and aside from indie gamers and some Smash fans she is pretty much unknown.