A bit late posting this here. I meant to do it sooner, and the weird spam around here is what finally reminded me.
https://avernale.tumblr.com/post/691624715975081984/have-you-seen-pok%C3%A9mons-latest-gimmick-have-you
I talk more about the presentation in the linked post, but to cut to the chase I'm not too fond of Terastallization visually. Mechanically, it's fine. You can switch your Pokémon to its Tera Type, which is either a different type or the same type with more STAB. People are saying it makes your Pokémon stronger, but I'm not seeing that explicitly expressed anywhere official. It lasts all battle, but you have to recharge it afterwards in specific locations.
All Pokémon can Terastallize, but rare Tera Types are mainly found in Tera Raid Dens. Basically Max Raid Dens, but you don't have to wait on allies while attacking. You can cheer even if you Pokémon is still conscious, and you can choose from stat-boosting cheers or healing cheers. I'm kind mid on it, since it's basically Max Raid Dens, but if it works, might as well keep iterating on it.
The forms themselves are dumb and gaudy. Your Pokémon turns crystal and puts on one of sixteen silly hats. It doesn't capture my imagination like previous gimmicks did, which is the main problem I have with it on Tumblr. However, I went back and checked the Dyanamax reveal trailer and it didn't have Gigantamax in it. I'm hoping the bespoke transformations for Terastallization (which I call "Ulterastallization") will impress me a bit more.
Using Wikipedia, I also discovered that these gimmicks are increasing in magnitude. Mega (Evolution) is a million, Giga(ntamax) is a billion, and Tera(stallization) is a trillion. Gen ten will likely have some kind of Peta Evolution (a quadrillion).
Also, "Teras" is Greek for "Monster," and I noticed there are simple eyes on the emblem on the silly hats. This has me wondering if "Terastal" isn't also the name of the Pokémon that governs this phenomena.
So, any speculation on what's going to be announced next? I'm still getting strong racing vibes here, especially since you get to ride a Legendary motorcycle Pokémon starting so early in the game. Or one that cosplays a motorcycle and runs around on all fours. Koraidon has failed to impress me, to say the least.