So, do you like the direction Smash is taking with "Videogame All-Stars" instead of just "Nintendo All-Stars"? We're probably just in the beginning of the transition but we have 10 3rd party franchises currently (11 if you count Bayonetta).
Metal Gear
Sonic
Pac-Man
Megaman
Street Fighter
Final Fantasy
Castlevania
Persona
Dragon Quest
Banjo-Kazooie
Mixed. During Brawl speculation I was annoyed at third parties because it felt like we were still waiting for a lot of classic Nintendo characters to be added, then comes this dude in a bandana. I thought if we got third parties they should at least have a storied Nintendo history, like Mega Man and Bomberman.
A lot's changed since then. I got into the MGS series thanks to Brawl, Ultimate turned me to the Belmont era of Castlevania games, and obviously from my avatar I played Persona 5 and absolutely loved it thanks to Joker's inclusion in Smash. And a lot of those classic Nintendo characters (Diddy Kong, King Dedede, Ridley, King K. Rool) got in. The third party door also brought home Banjo-Kazooie and I'm forever grateful for that. There's a lot of third party series I love, and almost all of them are represented in Smash Ultimate, which is insane to me. My top 10 favorite games all have playable characters.
But I am first and foremost a Nintendo fan when it comes to video games and I probably always will be. I don't mind the focus on third parties during DLC because I understand why they do it, and I feel like Sakurai hit the major obligations for first party characters in the base game (Inkling, Ridley, King K. Rool), so I'm not as fussed about it as I used to be. As long as the base roster for each Smash title continues to focus on first party characters with 1-2 new third party franchises, I'm fine with it. Characters like Isaac would be great but I understand why they aren't in the game with such a limited newcomer pool, something has to lose out. And really, if we're at the point where Isaac is one of the most requested first party fighters left, that really tells us how well Sakurai has done to include the vast majority of popular and iconic Nintendo characters up until now.
My favorite Smash character additions also tend to be ones besides the main character; other playable heroes, supporting characters, villains, and so on. Those additions really make it feel like a crossover between franchises, instead of just the one they slap on the boxart. As a lifelong DKC fan, I'm absolutely thrilled that I get to see the once-abandoned villain of the series interact with all these crazy characters; it feels like it's a genuine crossover with DKC instead of just the obligatory hero and sidekick. So if we continue to see more guest characters, it would be nice if they started to double dip. There are also some obscure characters I would love to see like Muddy Mole and Lip, but I'm not that invested in them, if we never get characters like that again it'd be an "oh well" thing from me as long as we continue to get good character choices overall like in the base Ultimate roster.
My perspective is different though. I got the two evil reptiles I've wanted for the last decade and a long-lost duo I never thought in my wildest dreams I would see on a Nintendo console again, let alone in Smash. I won. I'm not a diehard Isaac/Skull Kid/Waluigi/etc. supporter so I don't see other characters as "stealing their spot." So maybe it's just that I got what I want and I can afford to be detached. It's probably disappointing knowing you're waiting a long time to even have a chance to hope for those characters to get in again... but again, I got what I asked for. Even Piranha Plant's inclusion was a good wake up call to not take this stuff so seriously. Smash speculation can get very emotional, for better or worse. If your most wanted still isn't in... hopefully next time, but also, it's good to know at some point when it's become too taxing to stay invested. I'm forever thankful that the K. Rool costume had a happy ending.
The two characters left I would want to see the most are Dixie Kong and Bandana Dee. Above all else, even whatever crazy third party characters we might get on the Pass, those two would make me the happiest. I'm not sure what the future holds for them, but I'm not getting any younger and I already spend too much time posting about Smash online as is, and my desire for those two isn't so strong that I would probably stay active in the Smash community for the next game. If Smash 6 is announced in 7 years, and they suddenly decide to make the base game newcomers mostly third parties, I'd probably be a lot less upset than I would have been even a year ago. The Smash roster will never be "complete" but Ultimate is about as close as you could ever hope it could get.