I agreed, if Ash is staying for the Paldea anime then i hoping some of his un-evolved/middle stage mon returns and evolve like Bayleef or Gible etc. He can still mixed his team with fully evolved Paldea mon like Dragonite and Gengar mixing with his reserves that needs to evolve if they planning on Ash defending his crown now and then with different mon
With regards to your comment,
Butterfree: If we used the real life Butterfly dying after mating then i don't see it
Pidegot: I agreed with it and i suspect the random Pidegot flying up after Ash scene, i have a feeling that the writers did have plans to bring Pidgeot back until either they forgot about it or newer show runner turned down the ideas of Ash using reserves and Pokémon like Pidegot for the PWC.
Squirtle: I agreed too, they can announce his "retirement" as a firefighter due to him evolving to Wartortle and staying at Oaks lab.
Primeape: Agreed with you as well, hoping the new evolution will prompt the writers to bring it back hell they can write something like Anthony informed Ash that his Primeape somehow actually have an evolution but he feels that by having Primeape return to Ash and possibly joining his team for a while in Paldea might be able to triggers his evolution and ended up staying at Oaks after Anthony realized that Annihilape will fared better returning to Ash.
Lapras: I don't see it returning since it mostly likely swim to different part of the world by now and due to the dex entries on how the species is on the verge of being extinct, highly likely it not going to return
Lavitar: Agreed, Ash finally catch it and evolve to Tyranitar in the end
Ambiporm: this is the most surprising one like you say due to Ash owning her when she was an Aipom so having her appearing as Ambipom instead of Aipom was a surprise, maybe they can have one episode explaining how Ash got Ambipom cause unlike Mimey, they cant recon it offscreen back to Ash as everyone saw the episode of trading and Aipom evolving under Dawn care. How will you think the writer address the issue if it really do return under Ash.
Greninja: Might still happen if Diantha, the Elite Four, Gym leaders working together with Sycamore and taking over the issues of handling the vine from Greninja and he is free to return to Ash.
Goodra: i can only see it happening if the all of the swam mon evolving into their final form and convince Goodra that they can protect themself from now on if not no
Nagandel: No, like you slated about the portal
Solgaleo: Maybe Nebby agreed to join Ash with Lillie blessing and Ash status as World Monarch? (srry, never really see the ending of SOMO so im not really sure whether why Nebby didn't joined Ash in the anime}
Glad to know you agree with me at so many point, pal. Ash would definitely have to stay on his toes from now on to defend his new crown. Somebody once said "The king who sits on his throne for the first time is the easiest one to dethrone". Just let me add some things:
+ Butterfree: Real butterflies are not as bug as hawks, so your theory about dying after mating doesn't work here. I mean, Pokemon are based on real animals, but they don't live like them. In case they don't want to bring back the original Butterfree and ruin his "family life", there's also the Butterfree he befriended in the Decolora Islands. Although, let's face it, the Butterfree who appeared on Pikachu's vision had to be the original one.
+ Pidgeot: Yeah, I remember that scene from the episode where Goh captured all the Kanto's Bugs. I still keep my eyes crossed just in case.
+ Squirtle: Evolving won't be a reason to stop being a firefighter. Don't you remember Team Wartortle, from the Orange Islands? In any case, the old Squirtle Squad would finally mature to stop being such losers without their leader.
+ Primeape: Primeape could have seen Ash's victory on TV with Anthony, and if Primeape became a Champion by himself, he could decide it's the proper time to return with Ash.
+ Lapras: The thing about Lapras being endangered doesn't work anymore. I mean, there have been debuting several trainers who have a Lapras. One of them is a Galar's Gym Leader, whose Lapras can even Gigantamax.
+ Lavitar: He definetaly can't stay with mom forever, and since he knows a Ranger who protect the area where he lives, he might have access to a TV and now about Ash's victory.
+ Ambipom: Well, she didn't evolve under Ash's care, that's certainly. But that doesn't change it was the same Pokemon Ash took to Sinnoh and traded with Dawn's Buizel. Besides, when she left to follow her dream of Pin-Pong Pokemon, he said farewell to both, Ash and Dawn, as both of them have been her trainers.
+ Greninja: You know? Since Mewtwo is moving from one place to another to help Pokemon, I'm pretty sure his help would be fantastic to finally fix this problem.
+ Goodra: Honestly, with the Florgess and its allies now in the same side as Goodra and his friends, I don't see why they would need him to protect the swamp.
+ Nagandel: I agree with you. Still, I'm just going to keep the mind open, just in case. You never know, after all.
+ Solgaleo: I suppose Nebby didn't officially join Ash's team because, after becomign Solgaleo, he had a "mission" to fulfill or something. Still, after seeing how Goh captured Suicune and their relation (Suicune stays as Goh's Pokemon, but he will continue his mission and go wherever he's needed, returning only when his trainer needs him the most), he could become something similar for Ash.