So currently, all the pokemon utilize their following sprites from HGSS for their in game sprites, which makes it seem like adding the future gens would be unlikely as they dont have following sprites, but there is already fan-made ones for all of them!
https://reliccastle.com/resources/670/
This page is for the Gen 8 project for Pokemon Essentials, but part of it is the following sprites for all the pokemon.
Now, simply having them all doesn't mean it's easy to add them, as stats and moves still have to be determined for each one, but the the resource exists if the plan is to continue using these sprites!