I think that, as the roster is right now, there's still a few characters we'd need for every 1st party franchise to feel fully represented. Most do feel properly represented as is now, but here's a few characters i'd say are the most crucial ones that we're missing (list mostly in order of my opinion of importance):
1) Bandana Dee. Kirby games have shown for a while now that the main 4 characters are Kirby, Meta Knight, Dedede, and Bandana Dee. It's become a staple to the franchise at this point that if there's multiplayer, you will see those 4 characters in every game. So as it stands, the roster for Kirby feels like it's really missing that one more character to round it out. Honestly, any character added after to Kirby's representation would just be a bonus.
2) Dixie Kong. Same reasons as I feel with Bandana Dee, as Dixie Kong is a series regular and you're always assured to find her in recent DK games if multiplayer is a function. Sure you could make an argument for Cranky being missing too, but most people when they think of DK they think of the main trio of kongs that have been around for decades: Donkey, Diddy, and Dixie. With K Rool in as well, it would really make the DK series feel totally completed in smash.
3) Waluigi. Please don't kill me for this, but I think even for as big as the Mario series is represented, it'd feel really complete if Waluigi was in. I wouldn't necessarily say this if not for Daisy being in the game, as I feel if you have a game with Daisy in it to partner with Peach, you should have Waluigi in to partner with Wario. Sure, the only other character to rep for Mario that's a huge reoccuring character would be Toad, but I can atleast forgive Toad's absence with Peach's and Daisy's counters. Waluigi would really just make the Mario series feel complete in the world of Smash.
These 3 characters I would say have the highest importance to me as 1st party characters with series already represented. There are just a couple more that, while they may have series with good representation, could really help represent their series better than it is as of right now:
4) A Legend of Zelda Rep that doesn't hold the triforce. I would be fine with any character really. Skull Kid, Tingle, Urbosa, etc. It's just that the legend of zelda has so much more to offer from the series than just Link, Zelda, and Ganon, that I feel just one character that isn't one of those 3 would really better help the series representation in showing that there's more to it than just the triforce.
5) Ninten. With the 3 Mother games with 3 distinct protags each, it's always felt a bit off to me that we have Ness and Lucas to represent Mother 2 and 3 respectively, but nothing to rep that first game. An echo for Ness would be completely fine for me, and with how little time they'd need to develop the character, it would really just make that nice 1 2 3 representation feel great.
Honorable mentions would be Octoling, Geno, King Hippo, and a WarioWare rep. Octoling makes sense for Splatoon's crazy popularity, an echo would be just fine for that. Geno would be great as a highly requested character to match with Banjo, K Rool, and Ridley, although technically he IS a 3rd party character. King Hippo I feel is an iconic Nintendo character, and having that extra perspective to Punch Out would be nice. WarioWare rep I'd love to see because as it stands I don't feel Wario really represents Wario Ware as a series as much as it just does Wario as a character, so I think a character like Jimmy T, Mona, or Ashley would suffice to help that!
Finally, as far as other series why I didn't mention them, here's just a quick explanation for each: Metroid I feel we have just the right representation this game, feels great as it is. Yoshi I suppose you COULD throw in Poochy, but aside from that the Yoshi series is already super closely entangled with the Mario series with just how much shared the characters and world is. Star Fox I think feels great as it is, not sure what more different you could get with characters like Peppy or Slippy, and Krystal would just serve as a nice fan-requested addition than a fitting completion to the series. Pokemon is popular and has a ton of rep to support that, F-Zero can be summed up with Captain Falcon nicely, and no other character has as much memorability and recognition from the series as him, Retro characters are fine at one rep per series, Kid Icarus has a good amount of representation and maybe COULD have Medusa to make it feel complete (but personally I think it's complete as is), Olimar perfectly represents Pikmin as a character and anything added on would just serve as a nice cherry on top echo to him, Animal Crossing is good with the two it has now, Wii Fit, Miis, and finally Xenoblade i think could add Rex / Pyra for a good amount of representation.
For one last note, I will say personally I would love to see Golden Sun, Rhythm Heaven, and Arms represented in the future. Those to me are really the "biggest" Nintendo franchises that still have yet to be represented. Quotes on "biggest" because they'd be biggest in relation to what's yet to be rep'd as a character in the game.
Again, these are all just my own opinions, but at the end of the day I feel the Top 3-5 on my list would be perfect to make the roster feel even more complete than it does now.