Ash:
Pikachu
Inteleon - I'm certain that Sobble will evolve, seems fitting. Ash helps it out with its anxiety and Sobble eventually evolves to protect him and Pikachu. But that middle evolution.... both it and Grookey's, I doubt they'd keep them there.
Rillaboom - Not staying a Grookey, that's for sure. Least popular of the starters
Corviknight - Caught as a Rookidee, might be the first capture? But unless Ash catches it while they're in Galar(Most his bird captures were quick and not focus of the plot so...) in a few episodes, I think Sobble will be his first
Dreepy - It's a cute Dragon!
?????
Pokemon that I'd want him to get but have doubts on:
Obstagoon - Thankfully Marnie doesn't have one as her ace, what IS her ace I doubt they'd change as I have a hard time seeing that on the main cast... even though it'd be awesome!
Ponyta and Farfetch'd
Rolycoly - I was team Rolycoly from the start but the evolutions swayed me and made me doubtful if Ash'll get it.
Flapple
As for old Pokemon appearing? Let's wait and see, people are obviously getting too excited about the Bulbasaur and Snorlax episodes, but there's nothing saying he can't bring somebody back at some point. And yes, Melmetal does have a Gigantimax but if you remember from XY and SM, a lot of Megas and some z-moves never appeared in the anime!
I'm sure we'll see his Kanto starters reunite again, Greninja(Cause that Greninja is surely his) returns, and Gible evolving for DP remake promotion. But beyond that I'm not expecting too much, it'll be nice though if we actually got to see everybody! Especially if Kingler finally got a moment to shine and go Gigantimax!
Go:
Scorbunny - Hello marketable starter
Drednaw - Typical disobedient Pokemon
Something from Sinnoh
We'll see how they handle him, but I think a vast majority of his captures are going to be just that. No characterization, no permanent place on his team. I mean if they actually do let him catch a lot, that's going to happen. They can't give a personality to literally everything he catches
Boltund - Evolves late, I see them milking Yamper as long as possible and I don't mind that one bit! Can't hate dogs!
???? Honestly given the Mallow and Lillie situation, I'm not going to hold out hope she gets a second. I mean she isn't even a main character!
James:
Thievul - If not I hope Ash gets one
Polteageist
Grapploct
Jessie:
Sandaconda - She typically gets snake Pokemon
Thievul? I kinda see this one with Team Rocket in general
And this is probably highly unlikely... but I'd love it if the TRio wound up with Toxtricity!