Following Theorymoning, I do think Terastal will be broken:
-Not the defensive one that changes type (see: Cresselia, Avalugg, Blissey). They just change what beats them, but they are still beatable.
-Not the offensive one that change type either (see: Volcarona Ground, Barraskewda Fight or Psychic, etc.), since they lose the original type and just change their checks.
-However, at least in theory, Super STAB does sound broken. The checks are the same, but work way worse now, especially since the boost can be stacked with weather, terrain, boosting items or abilities like Adaptability too. So, you kinda need immunities or x4 resistances to even remotely check defensively EVERY OFFENSIVE THREAT. This is a short list of the Mons I think will have to be banned in order to preserve Terastal:
-Swift Swim, unless you pack Gastrodon or similar in every team, they will just break you. And even then, some might Terastal not in water type, but in the other STAB they have, like Kingdra or Mantine, whose Draco Meteor or Hurricane will demolish Gastro, Volcanion, etc. Blissey of course will probably still wall these, but outside of Stall it's not logic to pack Bliss in every team.
-Bolt Beak and Fishous Rend. Same as above, every single abusser just looks too broken if for some reason the immunity gets removed.
-Heatran. With the current Movepool I already consider this one broken, and despite losing Steel typing, I can't imagine almost anything that stops Fire Terastal Magma Storm if Heatran gets in. Opposite Heatran gets defeated by Earth Power, Tyranitar can get hit by Body Press, Blissey gets Taunted. Only Slowking and Rain teams are safe.
-Victini after Heatran gets banned. Band V Create demolishes resists unless they are supported by Rain. There are currently no good Flash Fire users other than Heatran.
-Tapu Lele. Currently there are already few non Dark Mons that can take 2 hits from both Specs Psyshock AND Specs Psychic. With Terastal in mind, the number drops to almost 0, so you will need a Dark (who arguably do take better non Stab Moonblasts) in almost every team and good prediction.
-Speaking of Darks, Band Knock Off from Weavile or Bisharp (who according to the leaks got an evolution btw) also sounds broken. Other Dark moves can be soaked, but Knock Off looks way too powerful if boosted by both Band and Terastal.
-Yanmega. This guy has Tinted Lens and even if it's usually unranked in OU, I have used it to some success in every available Gen. Not only Terastal will make that power higher, but also will remove the x4 weakness to Rocks, allowing Yanmega to come into the field more often. Flying Type has almost no good x4 resists (Magnezone is the only one, but it's not very bulky). Heatran (especially offensive versions) was already close to being 2HKOd by Specs Air Slash, with Terastal in mind Yanmega might even 1vs1 it (since Magma Storm won't OHKO back anymore). Bug Terastal also sounds very good, despite being a worse type both defensively and offensively, Bug Buzz hits even harder than Air Slash.
-Melmetal. Yes, this guy already suspected without Terastal can have Double Iron Bash even more powerful, blowing through the few checks it has.
-Porygon-Z. I think both +1 (from Download) and Adaptability + Specs + Terastal Tri-Atrack will 2HKO both Blissey and almost any Steel. Maybe Tyranitar does live 2 hits though, but running it and a ghost (who are still weak to coverage moves) in every non HO team is not reasonable.
-Basculegion. This has never been released, but it has Adaptability + high enough Speed and power. If it has a good Ghost Stab, I can't see it staying. Even with just Water STAB it could blow through every non immune Mon in Rain. Crawdaunt and Araquanid are even stronger, but at least they are slow.
-Kartana. That Band Leaf Blade now will only be stopped by x4 resists. Forget about Zapdos and probably a single crit will delete Tornadus-T from existance.
-Dragapult. Has more checks than the above Mons, but for far less power and with no coverage Spectrier ended up banned.
Of course, this is still Theorymoning and depends on many things:
-The exact power boost of Super Stab (it has been calculated to be x3 of No Stab though) .
-The non Dexited Pokedex (for example, Heatran and Galarian fossils have been Dexited according to leaks) .
-The expected movexit. If Weavile and Bisharp lose Knock Off, they are very far from broken even with Terastal.
-The possible new mechanics we don't know about yet.
Still, if Terastal is even remotely close in power to the predictions, I think it will be banned since it's easier to ban the mechanic entirely that to ban 20 Mons broken with it or a Complex (no Super STAB) Ban.
