Bit of math here, with a LOT of approximations : assuming, that there will be 900+ pokemon after SwSh are launched, 100 new pokemon will be introduced including legendaries and mythical, 400 of the 900 will be removed because they aren't popular enough, this will leave400 slots left for 7 regions(since 100 are
taken by galarians) to fit in their popular pokemon, which rounds off to 57 pokemon per region. That leaves a lot of scope for many pokemon to appear again.
I'm sure the Bagon,Magikarp and Zubat lines will be brought into Galar.
With that said, which pokemon do you think will make their way to the Galar dex ?
I think the Kanto evolutions and Hoenn, of course, will come back
I don't know why I think those 2 are the most popular bunch of the starters so they'll come back. or at least I think they will come back
Any top tier Pokemon that fans love will most likely make a return. Lucario, Charizard, Absol, Gengar, Eevee, Tyranitar... etc. I can't imagine them leaving out these guys tbh - though I wouldn't mind being surprised!