>>54829565The interaction of Z-Moves and G-Max moves, Mega and Dyna/Gigamax, and jank to arise from Terastalization would create a comprehensively unmanageable metagame. Retaining Mega Evolutions would be good form after they went two generations with them and had them cut off the "traditional" cross-gen evo help to a bunch of single-stage shitmons (pic related), but even then they'd made a point of denying Z-Moves to Mega Fug.
Terasalization has a non-zero chance to stay, as it's expressly NOT region-limited like Z-Moves (by handling of implement) or Dynamax (by Power Spots) were, but that might be stumbling back into the issues of Mega Evolution (where there's no storyline reason for removal) or be a clearly-aborted plan given the metagame issues and design constraints it poses that Megas didn't.
...It'd actually be interesting to have cross-gen gimmick fights. Potentially a bitch to code so incompatible with corporate's yearly release demand, but imagine you run into the gen 8 protag who decides to bring along Eternatus to MAKE a Power Spot in a particularly hard-to-reach mountain valley so you can face their Gigantamax option, or Game Freak throws in a Champion gauntlet and Diantha brings her Megavoire and gen 7 protag brings a Z-Crystal.
