"Generation 6.5" style: possible with Essentials 17?

I'm still hoping to turn my Pok?mon fangame concept into something I can release, but I've got a problem I hadn't thought about.

1. Generation VI has outside-of-battle HMs (which I want to keep) and Mega Evolution (which I also want to keep); but the only regional species variants are cosmetic, like Shellos coming in two colors.
2. Generation VII has real regional variants (which I want to include) and some other new material worth having; but Z-Moves, Ultra Beasts, and the Pok? Ride system don't fit the gameworld as I'm designing it.

Can RMXP and Pok?mon Essentials 17 create a game that's between Gen VI and Gen VII in terms of play style?

The way I see it, yes.

Maybe you'll be interested in these threads?


Gen 6 Project:
https://www.pokecommunity.com/showthread.php?t=314567


Gen 7 Project:
https://www.pokecommunity.com/showthread.php?t=387706

Gen 7 moves & abilities:
https://www.pokecommunity.com/showthread.php?t=379220

As each feature in Pok?mon essentials is coded separately, you can include everything you want and exclude what you don't want.
In the Pokemon PBS just erase all the ultra beasts entries, the icons of the ultra beasts in graphics -> icon, the battlers in graphics > battlers. That way you'll get rid of the UBs

Mega Evolution is already implented in Pokemon essentials, you just need PBS for the items, pokemonforms, and sprites.

In short you don't have that many work, as there are only the alola forms you want from 7g. There is nothing preinstalled in what you described.