I have a question for people. Since Sakurai is obviously trying to make each DLC character bring something new to the table and I think he's even claimed as much, do your most wanteds actually have the capacity to do so? For example, I know Rex talk has gotten stale, but I would like Rex (even though I'm unsure if Nintendo agrees), and one of his strengths is that Pyra and the concept of Blades provides some material for unique play mechanics.
My unpopular opinion is Crash, while iconic, I feel would probably have a basic b*tch moveset. I have a hard time imagining some kind of unique mechanic that hasn't been done belonging solely to him. If someone wants to prove me wrong, I encourage it.
The above test, I feel very strongly that Arle would excel at standing out from the crowd, mostly through chaining which I envision as both her greatest strength and weakness (you get stronger as you land more hits, and take more damage as more hits are landed on you). That alone would probably suffice, but in terms of the basics being a magician certainly doesn't help.
I have no idea where to start with Paper Mario. Guy could probably take up 5 slots on the roster before he'd run out of material to work with.
Even Frisk could have some form of mode change system where you can have them start neutral and go either pacifistic or genocidal, with the appropriate balancing (one is more geared towards survivability, the other towards making everything dead).
Ironically enough Lloyd is probably the character who I want most who struggles the most with this. The best I can envision for him is "combo Shulk" using EX Gems rather than some sort of levelling up system for his moves that doesn't really gel with how Symphonia handles things, but he's very much an "easy street" kind of character, very honest. Like you said, Crash is close, but I feel like Aku Aku could be that X factor, reducing the damage he takes until Aku's gone and then you've got one of the frailest characters in the game. You could also make his spin basically the Swiss army knife it is in his series.