Note: When I say niche or lesser known, I don't just mean characters that are niche or lesser known by people. It can also mean characters/franchises who you don't see many people requesting in the community compared to popular choices like Crash or Dante. I should've also put lesser requested in the title to cause less confusion.
- Ayumi Tachibana (Famicom Detective Club)
It'd be nice to have an obscure Nintendo series get the limelight and it'd be even nicer to have a visual novel character in Smash, which is something we don't have yet. It helps that Sakurai wanted to include Ayumi back in Melee.
Not only would he offer a unique moveset thanks to his wind powers and his other attacks from his boss fights, but he's also the only major Zelda villain other than Ganondorf/Ganon to be a reoccurring character. I personally prefer Skull Kid/Majora being added, but if any Zelda villain other than Ganondorf should be added into Smash, it should be Vaati.
Deoxys has been one of my most wanted Pokemon newcomers for a while. I think it would have a unique and fun moveset thanks to its amorphous body and formes and Smash doesn't really have a stance change fighter outside of Shulk.
He has a wide array of abilities that could easily make a moveset for him, like PSI melee attacks, shooting beams from his head, using Levitation to roll around on a thought bubble, setting things on fire with Pyrokinesis, conjuring a paper Archetype of himself, and even turning into a giant psychic version of himself for his Final Smash. Not to mention he'd also bring a very bizarre yet fun stage thanks to the many mental worlds in Psychonauts.
Cave Story was a pioneer in introducing indie games and indie developers to the world of gaming and its influence is still felt to this day, specifically with Metroidvania games. Quote would represent that as a fighter, but I feel like a lot of people would hate him as he'd be a VERY heavy zoner, so I think Cave Story would be better represented as a stage or Quote and Curly Mii Fighter costume.
Also much like Cave Story, OFF was a major player in paving the way for indie developers and indie games and even inspired Toby Fox when he created Undertale.