I thought the idea of “no moveset potential” was laid to rest when we got a Piranha Plant as a fighter. Im sure the team could find a way to make Pauline work.
That's... a lot more believable than some. There's a ton of PP species with tons of moves. Of which Plant actually takes from too.
Zelda's a better example of this situation, since she barely used any of her own abilities(or any of her counterparts, like the one from the TV Show, or the CD-i games, or ALTTP, etc.). She was far more made up than Plant ever was. And even with the few Goddess spells and her respective Grab, it's still really loose. Sheik is similar, though her move count is... actually the same for "things she does"(Transform and Vanish are both normal moves for her, whereas Zelda only could do the Grab and Transform). Only difference was that Sheik was easier to imagine due to being a ninja archtype. She didn't need to borrow any items from Link just to make any kind of moveset.
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That said, I think we also need to keep a lot in mind that it's not simply moveset potential alone. It's "how can we make them interesting but actually feels remotely fun to play as." While some oddballs are in, a lot of their animations(like Wii Fit Trainer) do actually look like moves capable of dealing damage. Villager and Isabelle have a literal Axe they use(though the Bug Net is the one you can actually hit a normal character with in Animal Crossing). That's not saying there's an issue with other oddballs who don't show anything that kind of looks like an attack move either. It's just that they stand out compared to characters like that as well because they do take from more believable attacks.
Something like Phoenix Wright is absolutely awesome as an idea, but he wouldn't really compare to them. His MvC moveset is essentially weird moves that don't resemble being in a fighter. They're not supposed to. They're supposed to be silly and represent the style and craziness of his respective gameplay. They do not look like fighting moves(something that every Smash character does have). That's not saying it's a problem either. I note again that "it just simply means they don't compare that well" and nothing more. He's not one of my favorite options, but certainly would be cool too.
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As for something else;
Smash 4 is honestly the one CSS that broke logic. It wasn't so bad with the DLC and Clone Trio, but combined with Yoshi being in a completely bad spot(why isn't he after the Mario characters?), it was very clear that it does not actually follow franchise logic. You can somewhat justify Yoshi and the Clone Trio, but then the DLC hit and ignored it entirely. Previous games didn't. Ultimate doesn't use that logic(bar Echoes specifically), and that's fine, but 4 really needed a better CSS overall. Not being able to fix it yourself didn't help.
It does somewhat help that DLC was not originally planned either. It was just Mewtwo at best, as a bonus character. Unlike with Ultimate, where PP was the first planned DLC character(and had a First Pass plus potentially 1 or 2 more characters ideally after that. Pass 2 came later). So they already may have had a decent way to fix up the CSS with that in mind. There's also a question of why Yoshi, from a different franchise, is weirdly in that spot. It's not just cause of "who appeared first in the franchise" either. We already know Bowser Jr. barely made it in, meaning that Yoshi may have been the final one before DK/Wario's franchises. Was Rosalina a later idea, perhaps? Was the CSS already partially started? Or was it just a bad decision to break logic for the sake of it. That said, either way, it's a pretty badly designed CSS. Not following any clear logic really didn't help. And it most definitely can't be called a case of "grouping franchises together" when it split them multiple ways.
Ultimate at least had a reason for the split, with no strange/bad decisions to justify it. It's hard reveal order. No ifs, ands, or buts. Smash 4 meanwhile has some reveal order at times, some franchise at times, last minute additions, Mii Fighter, DLC, and a completely asinine split that we can't possibly figure out why(because it's not actually obvious enough, just easy to make theories on at most). While you can decently justify the DLC not moving into other places on its own, it would've been very easy to just keep the clones next to their counterparts, have Yoshi right before Wario, and call it a day. It would've looked a lot cleaner(that, and the DLC also was its own tab on the 3DS version, so it wasn't really an issue there). ...Also, the fact the Mii Fighters were still at the end in the Wii U version after the DLC fighters further showed how completely disorganized it was. They should've at least been before Mewtwo(which is technically the case for the 3DS version anyway).