-Parin from Gurumin
Gurumin is a fun platformer/adventure game that's probably slighty inspired by Zelda; Parin uses a drill staff as a weapon. The gameplay of the original game lacks some polish but there are many interesting concepts that would be cool for a Smash moveset; from the rhythm based critical hits, to the level up/down of the drill, to why not, even the ability to summon the monsters. Parin already has a complex moveset in the original game, that with some tweaks can be adapted to Smash very easily IMO.
-(since 4th parties count) Hamtaro
I know it seems ridicolous to think Hamtaro as a fighter, but well... I recently (I admit, it was initially just for the lol and for the Master Sword cameo/parody) played the Game Boy Color/Advance trilogy (there are more games but only 3 of them are adventure games), and I was surprised, especially by the first two games. Granted that they are obviously games for young kids, they have some nice original concepts, they have some consistency and identity as games, even ignoring the existence of the anime (they're not really connected to it aside of using the same cast of main characters - hamsters only, no humans). think of some graphic adventures such as Monkey Island and similar; instead of collecting and combining items, you have those "Ham-Chat", that's the hamster language. In order to interact with the NPCs you have to learn those words by putting other hamsters in specific situations, and the game is a huge puzzle where you need to experiment with those words in order to find a way to make progress in the story.
Each word also has a specific animation, in fact they can be used to compose a dance in a minigame, but at times they are also used to physically interact with the world. For example, a word that means "enemy", features Hamtaro with an angry face and a big flame behind: you can use that flame to light up an actual fire.
The third game is less interesting IMO because it's mostly a collection of minigames with a story and a world map, but it introduces the gimmick of working together with other hamsters as a group, where each one has a different skill, and you have to build up the party by selecting the most useful characters from time to time.
Can't help but think about all those gameplay elements as a huge inspiration for a very unique and hilarious Smash moveset.
Wouldn't be epic Hamtaro vs Pikachu?!?