I can think of a lot of characters I'd like personally that would be challenging to implement in Smash, but have a lot of potential to be something very unique.
However, none of them are very popular or relevant lol. But I will say that I'll mention Kawase Umihara in response to this question.
Basically, her games are physics-based bungee-cording platforming titles. She uses that fishing line to grapple to almost any terrain and has to use momentum to traverse the level.
This clip basically gives you the gist of some of the nonsense you can do the games, but there's a lot of speed runs of the several games that can really show you what is possible.
Umihara Kawase (SNES)
Umihara Kawase Shun (PlayStation)
Sayonara Umihara Kawase (3DS)
Kawase also appeared in Blade Strangers, which is a more traditional 2D fighter. There are two versions of her in it (Think Ryu and Evil Ryu).
Here's a combo video of the two of them if you were interested in seeing how she's been adapted into a fighting game.
Kawase in Blade Strangers
Summer Kawase in Blade Strangers
Anyways, I just think she would be really cool and interesting in a game like Smash. To balance her insane mobility potential her normal stats would probably have to be mediocre to bad, but if you can master her Fishing Hook techniques I think you'd be more than able to make up for it.
OH, and if you're at all interested in what you're looking at, Umihara Kawase is on the Japanese SNES Online. SO if you pay for online just download that and gave it a try. There's isn't much dialogue in it to worry about not understanding, but the game does leave you to more or less figure everything out by yourself.