As a prelude, I don't know what the policy is on discussing aspects of the leaks that have proven accurate but haven't had official confirmation, which impacts the subsequent point indirectly.
As far as Tera Types go, the thing I'm curious about is if they're hard-set on the Pokemon once it's generated. Comparatively, Pokemon games have been going back and forth with mechanics and means to change or circumvent otherwise "locked" parts of a Pokemon's battling profile, such as Gigantamax Soup, Ability Capsules, Hyper Training, Mints where Nature stats are concerned, etc. Will the games end up offering ways to tweak a Pokemon's Tera Type, however effort/resource intensive it may be, or will a Pokemon be completely and definitely sticking to the one it spawns with?
I could see particularly versatile Pokemon who might run sets that are fairly similar between variants outside of changing their Tera Type for coverage, and the ability to alter that might be the difference between raising multiple instances of a Pokemon to try different teams out for PvP.
I'm mixed on the designs of the new Pokemon, but cautiously interested in the Gameplay direction promised and the way the new Gimmick works. While still a distant second, it's the most a generational gimmick has interested me since Mega Evolution.