You're right. Maybe Melee/Brawl Zelda/Sheik is more fitting. I think Wanda and Cosmo are different enough to have a different moveset. Hell, throw in Poof in the mix and you'd have a Pokémon Trainer kind of fighter.
I'd obviously want Timmy over the fairies, but I wouldn't mind this either.
Just let Timmy and the Fairies be a single character. Not everything needs to be forced into a specific duo.
come to think of it that could also play a part in why we don't have Timmy yet. If they all function as one character that's a lot of models to animate and design, not to mention having to implement the FOP artstyle in particular
Can't imagine it being as much of an issue as Rex & Pyra were to Sakurai. The trio could all use the same animation skeleton, some minor animations could be shared between Cosmo & Wanda, and their designs lean more towards Popo & Nana's simplicity than the busier designs of XC2.
I think you guys might be over complicating this just a bit. If they decide to make Timmy, Cosmo and Wanda a single unit, they easily could. Here's how I picture it.
You, the player, control Timmy. Miniature versions of the fairies float around behind him. Their animations mostly amount to waving their wands when granting Timmy a weapon, panicked expressions when Timmy is hurt or grabbed, and vanishing in the trademark POOF clouds when Timmy is knocked out.
Maybe there could be a mechanic where you can switch which Fairy is providing the "wishes" and thus have CatDog style variations of moves, with Wanda's being the safer, more reliable options, while Cosmo's are more high risk high reward.
We don't have any group characters who act as two separate entities yet like the Ice Climbers do (Youre technically playing as Stimpy with Ren riding on his back, not both Ren and Stimpy at the same time), or characters who literally swap models and movesets (CatDog mostly just turns around, since they're one body) and given this game's limitations, I don't see these things happening anytime soon.
Which means I've gone from rooting for solo Beavers to being absolutely sure of them, hah.