Relic Castle

I'm still hoping to turn my Pokémon 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 Poké Ride system don't fit the gameworld as I'm designing it.

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