Added Steel Diver and The Wonderful 101 (can't believe I forgot that one) to the list.
In the case for Marshal, yes, he seems to be like the Wario (WarioWare) of Rhythm Heaven Fever, but by research, I only see Fever as the one he was in. There were other characters that looked like him, yes, but he himself was only in Fever.
If Rhythm Heaven were to get a character, I would bet more on Karate Joe, as he and his game were featured in all of the Rhythm Heaven titles so far and he's clearly fighter material, being a martial artist. That being said, I don't think Rhythm Heaven is going to get a character, and it's not for the "nonsense" about the characters being unable to fight (as again, Karate Joe). It's the fact that there's really no one to add that makes absolute sense from a logical standpoint.
I'm not saying the series hasn't done enough to warrant a character, but I'm saying characters shouldn't be added willy-nilly just to give such and such a series a character.
This is also why I don't think Fossil Fighters will get a character. From a logical standpoint, there's little to work with that makes absolute sense.
There is the Pokemon Trainer route, and go with the original Fossil Fighter (named Hunter in the manga), but that would just rip off Pokemon Trainer's gimmick but using Vivosaurs. Speaking of the Vivosaurs, we can't even go the Pikachu route and add the mascot, T-Rex, as the it's a frigging 50 ft beast of destruction (well, 42 ft, but still). At least Ridley has been small before and changes size practically every game he's in.
And even the starting Vivosaur of the original game, Spinax, is rather large (29 ft).
Pretty much, all the notable Vivosaurs are either large, not the right build for Smash, or both. Save for maybe the legendary chicken Vivosaurs which are around Lucario's size. Though a random chicken from a series that is about dinosaur like creatures would seem very strange.
And another thing, despite the game featuring Miis as main characters, Pilotwings Resort is part of a series which had main characters in the other two titles. Most notably Pilotwings 64 (as Meca Hawk, a robotic doppelganger of the character Hawk from 64, appears in a cameo in Resort). Lark (the supposed main character of 64) would be an interesting addition to Smash. The likelyhood of that happening though is quite slim.