If we are talking about the Nintendo side of missing content I would say:
the absence of Mike Jones and Nester is strange and it might be because they aren’t Japanese Nintendo characters
Virtual boy content has not yet been represented properly
Wario Land still lack content
The strange absence of Porky
Pokémon mystery dungeon lacks content
R.O.B. Doesn’t have a stage yet
Yoshi and F-zero still not getting their second fighter since the original smash game
No custom moves other than Mii Fighters
Nester was from the Nintendo Power Comics, which NP is defunct. It might just be rather low priority. Basically in the same area as Captain N. Notable, but aren't really game-related. Mike Jones would be nice, though. Even as a Spirit alone.
...Wario Land does have small bits of stuff, but needs more, yes.
Porky may not be missing. The two Capsule designs are almost the same. It's a very easy mistake to make for anyone looking up the sprites. The fact they used a sprite from a Deviantart artist instead of the official one shows they aren't perfect.
True.
Depends if they can find a unique area from the few games R.O.B. is in.
That's a bit more justified. F-Zero is beyond dead as a series, and doesn't really have much options outside of quick clones. And Falcon already has one. Yoshi is barely its own thing beyond stages and music. Every single option are also from the Super Mario games, making it... pretty awkward. It was just luckily its own symbol at this point, but isn't branched off enough.
Custom Moves take a massive amount of work. That's why they're only for the Mii Fighters now, since that's their core gimmick(their character concept isn't really a gimmick in the context of unique costume options). Palutena was the only one other to have unique ones and they just took her various moves and put them together to try and create a new balance.
I'd say Toad as playable is definitely on the missing side. Captain Toad isn't in the same all-star status, but having little content is still odd. A stage cameo and some spirits isn't really enough. It's a very good spin-off IP that should have more stuff in general. Also, there's no real Luigi's Mansion 3 content beyond a literal Poltergust design, which is normal that characters take cues from games coming out recently/soon for their designs. So it's badly represented.
Could've used a lot more general Nintendo or otherwise new stages. If they hadn't made the stages from scratch instead of porting them, that might've been a bit better. The stage list isn't technically lacking, but it does feel lopsided. There's less than 10 new stages until the 11 from DLC. Not even 20 new stages for a game with over 80 characters does feel strange, imo. And the newcomer list alone is pretty reasonable sized. They're usually 10-20ish on average, so it doesn't feel odd.
