Personally, when I think of characters that actively feel missing from Smash's current roster, a few come to mind. I guess I'll organize this into groups.
The first group, and easily the most "must-have-y" of them, is from relatively major but unrepresented series. Golden Sun, Rhythm Heaven, and Murasame Castle were formerly in this category, in my opinion. With those out of here, the ones that feel the most missing are
Chibi-Robo and
Advance Wars. Especially Chibi-Robo, I think, but that's probably bias. Maybe
Starfy as well, though that's got a bit of a regional thing to it. It's unusual that these are all GBA and GameCube-era series, but I guess the release dates of Smash just kinda cause that to happen.
I'd also like to shout out
Lip. Her wand has been an item for four games now, and she's
constantly kicked out of her own series. I just feel bad. Give the poor girl
something.
There's also
Ayumi Tachibana, if only because she'd gotten considered all the way back in Melee and is now relevant again due to the remakes. Though at this point we're not
The second group is secondary characters from currently-represented series. There's
a second Sonic rep, now that Sephiroth's in.
Dixie Kong's gotten talked up a lot. We're overdue for a
new unique Zelda character, even if we did redo Ganondorf. I think no character, however, shows the difference between "must-have" and "hype" more than
Toad, a major character that would probably not excite that many people.
The third group is unrepresented genres, which is where we get a lot more general. We don't have a
puzzle rep; maybe Lip could be that. We don't have a
visual novel rep; maybe Ayumi or Phoenix Wright could be that.
So there's a few character's I'm seeing here that I really don't think qualify as must haves, and are more of just "I really want this character in". Let me put it like this:
Imagine you had 100 people in the same room who were on the fence about buying a new smash game, and you were going to tell them who the last base roster slot was going to. Anyone who isn't convinced to buy the game by this last character leaves the room. If you wouldn't feel confident that at least 60 of the people in that room stays when you tell them who it is, they should not be a part of this discussion.
That isn't measuring the must-have-ness of a character, that's just measuring how exciting of a pick they are. Sephiroth would win this test, but I'd struggle to say that Ultimate was in any way obligated to have him.