I personally think the remakes are not long for this world. Will they actually do Unova? Possibly, it may be the last of the remakes. In the last three gens (Alola, Galar, Hisui, Paldea), we have seen many of the Unova and Kalos Pokemon brought back in many ways.
In the main series games, we have had significantly larger Pokedexes than we had in the first three gens, and it’s been something of a “greatest hits” set of choices of older Pokemon reappearing in these new locations.
That’s the direction I think we are going in. Eventually we might not get remakes but we will get wider and bigger worlds with many, many species included.
Paldea feels quite big. Breath of the World‘s Hyrule map is about three times larger. We’ve had hints towards an Australia themed game in future. Going proper, full on HUGE map is in our future for the main series games, and with it we are likely to get a Pokédex of well over half the existing Pokemon (600 or more).
So the reasons for remakes gradually fade away. Bigger main games that are more ambitious in size and scope will provide ample opportunity to catch all of those older Pokemon.
There is one interesting thing though. So far, Paldea has absolutely bucked the trend on reintroducing older legendary Pokemon. Sw/Sh did this, as did US/UM and S/M, and ORAS, but in Sw/Sh‘s case they were part of a DLC. So Paldea may yet give us opportunities to get the older legendary Pokemon again.
Mythical Pokemon are the preserve of events, cross game unlockables or Pokemon Go now, it seems.
I think the constant clamour for a return to Kalos probably doesn’t recognise that no new series of games in the Pokemon series is going to undermine its own new development. It was of no surprise to me that the DLC wasn’t Kalos themed. It was never going to be. Paldea is the main event, same as Galar was for Sw/Sh.