My personal opinion on the rosters on the whole '45 veterans + 15 newcomers + 5 echoes' is that across the board...people are a bit too lenient to the 3rd parties. Because let's be honest, even if we discount the echo fighters, and even if we consider the PT as 1 fighter instead of 3, it's still an 80+ roster getting cut to the bare minimum, and in such a tight window, I genuinely don't think anyone not named Sonic, Mega Man, Pac-Man or Ryu is getting in. That's not really because companies are easier or harder to work with it's just that... they won't be sought after (for base roster always). And also let's not discard the obvious fact that the new game will introduce new 3rd party fighters, like every game since Brawl has, with Brawl introducing 2, 4 introducing 2 base roster and then 3 through DLC, Ultimate introducing 3 (though 2 being echo fighters) in base roster and 8 through DLC, I don't see how the base game of Smash 6 doesn't bring let's say 2 new 3rd parties to the fold. But with already aforementioned 4 and the 2 debutants, that's already 6 3rd parties in a roster of 65. I don't see how a 5th guest vet returns in that window, again, it's not a matter of a company being difficult to work with but moreso that Nintendo/Sakurai won't seek to bring back the guests over their own characters.
I guess another smaller nitpick I have is that 15 newcomers, echoes not included is a lot for the new game. I think about 15 but with say 2-3 new echo fighters is fairer to assume.
With that said, I guess in this hypothetical scenario the veterans I'd bring back are:












(the original 12, I know that people are iffy on Jigglypuff, but imo, after 5 games, their importance to Smash Bros has only grown including hers, we can argue about more extreme scenarios on some of their chances but even among 45 vets, all 12 have to stay)






(these are all Melee veterans with perfect attendance but also pretty significant characters for Nintendo, Peach and Bowser are obviously huge icons, same for Zelda and Ganondorf, Marth is the face of Fire Emblem and is clearly valued by Nintendo/Sakurai and Mr. Game & Watch is the most significant imo out of all the retro picks because he represents the most important part of Nintendo - their beginnings. Basically all of these are just as safe as the OG 12 imo)







(these are Brawl newcomers who also feel big and they are all either either new franchises - Pit, Olimar, kinda Wario - core characters from big franchises - King Dedede, Meta Knight, Diddy Kong, kinda Wario - or some of the most popular and significant characters in their franchises - Ike, Lucario, Charizard - or just very good at representing their franchises - Pokémon Trainer -. These also are staying imo)



(these are the Smash 4 characters that I'd bet my money on being future mainstays, once again they either represent current big franchises for Nintendo - Animal Crossing, Xenoblade Chronicles, Punch Out -, significant games - Robin - or super popular picks - Greninja - I think they roster cuts would have to be extreme for us to consider cutting at least 1 of these)

/

(these are the Ultimate fighters that I consider to be mainstays going forward. Inkling and Isabelle are the faces of the 2 biggest new franchises for Nintendo, Ridley and King K. Rool were both super big popular demands and I just can't imagine them getting cut just 1 game after their big debut and Pyra/Mythra - which I count as 1 fighter - offer more xenoblade representation and are the most popular characters from the most popular game)
So that's 37. And with the aforementioned 4 3rd party characters (



- all of which are the ones that due to how big, significant and popular they are feel like they have to stay) brings us to 41 vets. Now for the other non-echo 4 veteran spots, I'll go with
. The Ice Climbers I think are just really valued in Smash and are also very unique, and I just know that at least a 2nd retro rep will be back and the Ice Climbers being specifically chosen for being a retro rep and being unique (as well as their return to Ultimate being treated as arguably the biggest deal among the returns), makes me very confident that they'll return. Byleth is from the most successful modern Fire Emblem game and brings moveset variety and much as the fandom would hate it, Fire Emblem as a franchise won't get completely gutted come next game and Byleth is besides that a worthy addition, so I'd bet on him staying. Rosalina is a very unique fighter, covering a whole archetype by herself, and a very popular Mario character and hailing from one of Mario's most successful and acclaimed games and establishing herself as a main character warrants her a spot. That and I'd say Mario is too big to be reduced to the bare minimum and Rosalina feels like a more notable addition than Bowser Jr. I'm pretty confident she stays over him and in general I think she is rather unlikely to be cut. Min Min... I guess is the most controversial pick here however, I think that ARMS being one of Nintendo's newest IPs and by all accounts not being a flop in addition to her unique playstyle would get her a spot and I think it'll be controversial but again, ARMS for all we know might get another game and is a Switch franchise so I can see Nintendo/Sakurai wanting to keep it in the roster.