People weren't defaulting to hating Byleth because they were first-party, but rather what kind of first-party they are.
Most likely - but I really believe people wouldn't have been as receptful of first parties from existing series like say, Dixie Kong or even Waluigi as they'd be considered just less "hype" in comparison to the immense clashings of videogame giants with their mascots or iconic reps.
I'm also reminded of sizeable portion of Smash's fanbase still thinking Banjo was an overhyped character with a boring moveset, having only been made notable and pushed by "loud middle-aged Ninty-fans that constantly talked how amazing the series was in Youtube".
Furthermore, there's more focus on third-parties in DLC anyway because it's more profitable for the IP holders than having them in the base game.
Perhaps - but the way it ended completely shifting the overall perception of what Smash Bros to it's playerbase and outside of it, to be now considered "The Celebration of Gaming - The Game" - has only cemented that as most favored direction for the series for majority involved. Would mean steady stream of big profits and major marketing for the IP-holders too.
Putting these two points together, I really feel the focus on 3rd Parties will be more heavily favored in the next installment, as it seems to Sakurai, if staying a supervisor or even a director still, that his series is looking to be scraping the bottom of the barrel in means of roster picks after the mandated pick for Pokemon, foreconcluded inclusions of Fire Emblem Engage + Xenoblade 3-reps, and maybe one rep for Metroid and Zelda each - incase he's not willing to explore recurring long-wanted characters he has traditionally pushed to NPC-roles before.
I guess this is why I'm really hoping for Sakurai to pass his directorial seat to someone else so we can finally explore this series with fresh eyes and blood, perhaps being less averse to these enduring fan-favorite picks - like Waluigi, Bandanna Dee, Dixie Kong, Captain Toad/Toad, plus handful of obscurer, Assist Item-bound Ninty-reps like Isaac, and so forth. Though we could just end up looking getting more creative never-knew-we-wanted types of characters like Plant or Wii Fit Trainer. Even if I don't share the same opinion.
...quite frankly I kind of feel like they should greenlight more DLC for Ultimate just because the fanbase is now focused on what DLC is like, rather than base game newcomers. Even those who believe more DLC for SSBU is completely and utterly impossible are focused on who'll join the next game's DLC, which makes speculation feel like Schrodinger's FP3 discussion.
I might see this as a possibility incase Smash Ultimate being ported to Switch 2 as either the bridging point between a more conventionally shaking Super Smash Bros. 6 - but I think we had hit the ceiling with the hype by now due Sora. Unless we really need to bring in Reimu Hakurei and maybe even that Fortnite-guy to break it even further. Also maybe have Master Chief on the side, unless we get Dante or DoomGuy be next chars to be upgrade straight from Mii Costume to playable in the same game... sorta.
With SSB6's case, I feel the base roster then would mainly end consisting of the fore-gone picks and maybe a few newer chars for existing franchises.
Having observed how Ridley and K.Rool (and most likely Banjo) ended mainly becoming a reality through the Smash Ballot, while some first-party or 2nd-party chars still long-rallied by certain fanbases got still passed over and/or continue on their given NPC-roles in the series
(Bandanna Dee, Waluigi, Dixie Kong, to some lesser extent Toad) --- with some even being brought back just to do that such as Isaac... it has me feel the chances of vocally wanted, "fanservicey" picks from Nintendo's side will be really slim next time around.
I mean, we might maybe get 1 or even 2. All I'm saying is that any "highly requested" picks just haven't been exactly of importance or worth to Sakurai since SSB4-days, explaining why we got a lot of them glossed over or turned to NPCs (Ridley's whole internet fame before Ultimate) - whilst going for more unorthodox or not-as-"obvious" picks from Nintendo's recent or profitable franchises, usually in virtue of finding an interesting moveset or playstyle in them.
I don't really think that Goemon has null chances to be in Smash, i mean, yes it's one of those things that we should not be expecting but i don't think he's impossible considering he was a Mii Costume, Sakurai seems to really like his franchise and after all he's still fondly remembered as a retro character in Japan. Honestly i sometimes think that Goemon is one of those characters that should be in Smash Bros, like my god, he has more history with Nintendo than most of the third-parties that got in Ultimate...
A series of humorous titles in various genres by Konami starring Goemon, a warrior in feudal Japan who sets out to fight evil. The character is based on the historical Ishikawa Goemon (石川 五右衛門), a...
www.mobygames.com
I frankly think Mii Costume is
all Goemon gets going onward.
The series has been in total purgatory since 2005, and since then, he's only been relevant in pachinko-games and/or being a designated retro guest character/cameo in other Konami's games - with ones related to Bomberman having grown to 3 by now. Doesn't help that Etsunobu Ebisu has long realized a spiritual successor to it in Bakeru, which has gained an English-release lately.
Maybe Goemon's chances aren't "impossible" - but they're still astronomically low, especially next to Bomberman and already repped Castlevania + Metal Gear Solid, which have moved long on from where Goemon had been left of, and still see pretty strong relevancy. Castlevania could use a major new title though.
Contrasting that, I feel there's never been more fortunate and beloved guy than Goemon and his series as it gets to live on through fan-translations, especially with Smash 64 Remix putting him to spotlight for people to see and go try his games through - now that majority of mainline ones have seen an English translation up to PSX and N64.
In anycase -it'd be better to just hope that maybe in the future, Konami has the guts to explore bringing back Goemon as a core IP and kickstart a revival for it like they did with Getsu Fuma Den a while ago. Would feel more gradual and confident from their end and show they're not seeing it as some retro IP solely for cameos anymore.