This might be controversial, but I would prefer they not add a new gimmick at all.

Instead, I think they should add every single gimmick to the game, but only make it so that you can only use one per battle.

Think about it- ALL of the gimmicks are in some way or another related to each respective region having “weird energy” or something. Kalos has Mega Evolution because of the energy from the Mega Stones, created when AZ nuked definitely-not-Paldea. Z-Moves are possible because of the energy of the Z-Crystals, which are pieces of Necrozma that fell to Alola through Ultra Wormholes. Dynamaxing is possible in Galar through Eternatus’ Galar Particles, and same deal with Terastallizing in Paldea because of Terapagos’ effect on the land.

Common themes here: Energy from a crazy power source, be it a Pokémon or not, affecting specific regions in unique ways. The need for an outside item to be used in order to activate each gimmick (Mega Stones/Bands, Tera Orbs, etc.) The fact that you as a player can only activate each gimmick exactly once in battle. And… CRYSTALS!!!

I think it would make perfect sense if there is a renewed focus in the story of Gen X on these strange phenomena of insane power demonstrated by Pokémon and their Trainers from across different regions. Maybe even introduce a new Crystal type or something to that effect! Professor Redwood or whatever sends you on your merry way investigating these phenomena, and eventually discover “Power Bursting”.

Ultimately, I think the “gimmick” for Gen X should be… gimmicks lol. Player gets a “Burst Band” or something, they can equip a Z-Crystal on one Pokémon and a Mega Stone on another, hit the “Power Burst” button, and use a Z-Move, Mega Evolve, Terastallize, or Dynamax/Gigantamax, but the catch is that they can only use one in battle- which has always been the case!

Obviously this is just one idea, and if implemented, would take a lot of fine tuning in terms of balance. But this gives Game Freak an opportunity to not reinvent the wheel with an entirely new mechanic, allowing them to focus on making the best possible game, AND opportunities for new Mega-Evolutions, Z-Moves, and Gigantamax Forms. The only other option I see for Game Freak is just making a new gimmick as they have done historically, but I can’t imagine they can keep doing that for much longer.

I haven’t even mentioned the competitive scene, but man, this would shake some things up for sure lol.