64 - Falcon and Ness. Arguably Samus too since she hadn't had a game in 8 years by her Smash appearance, IIRC, and a lot of people didn't know who she was.
Melee - Ice Climbers, Marth and Roy, and Mr. Game & Watch
Brawl - Ike (his games sold TERRRRIBLY), Pit, and Lucas. Arguably R.O.B too.
Smash 4 - Shulk and Greninja (Pokémon is mega popular but the fact is there were FAR more easily recognisable Pokémon they could have chosen if "the most popular/sales effective" choice was a factor. Not even counting Mewtwo here).
F-Zero was not niche. Captain Falcon yes, F-Zero no.
Mother I already covered.
Metroid also wasn't niche. I think you're confusing "niche" with "not one of Nintendo's top 5 most successful series" here. And most people
here might not have known who she was prior to Smash, but that's because people
here were really young if even born when Smash 64 came out. In 1999 most gamers knew who she was given that many didn't start gaming in the 5th gen.
ICs/Game & Watch/Pit/ROB - not talking about retros. Already covered that too. They're in their own category, one which Xenoblade isn't.
FE wasn't niche in Japan. Japan is who they were added for. Read above, 770k solely in Japanese units on one title plus an anime to boot does not niche make.
Lucas/Ike/Greninja - not talking about existing series, again something I already covered. Xenoblade was not an "existing series" it doesn't fit the paradigm
you're advocating exists. Also trying to argue that a starter Pokemon is niche is kinda funny. No starter Pokemon is niche.
And Xenoblade I certainly already covered. Niche, yes. Many reasons to include which STEAM and the vast majority of other niche titles do not possess, also yes.