To add to my previous post; Lower-tier mons, Ice types, Rock / Steel, or Rock / Dark hybrids could be quite a bit more viable (in this new meta because they can make themselves immune to Fighting-type attacks with Ghost tera type, or they could resist Rock with Steel tera type, etc). Imagine something like Stakataka being immune to 4x Fighting, or Linoone setting up Belly Drum Extremespeed with no weaknesses, or Volcanion becoming immune to Ground attacks with a Flying tera type.
Hell, Zygarde-10% could be viable in OU with something like a Fire tera-type to get rid of its 4x Ice Weakness. People aren't getting the magnitude of this. Tera types are more a more significant gameplay change than Megas, Z-Moves, or even Dynamax / Gmax are. Frankly, from what I've seen of this, it could be the best thing to happen to comp Pokemon in a LONG time.
There is so much room for variety, experimentation, and creative teambuilding I don't even know where to begin!
There's no question that lots of Pokemon that would love to change their type, but I think you might be looking at it the wrong way for some of the terrible Type combos. Part of what makes some Pokemon with bad types okay or even good is their STAB and Terastallizing to the same type lets you buff one STAB at the "cost" of losing the other type. Aggron and Stakataka would love to buff their Head Smash and Gyro Ball on offensive sets and Zygrade's Thousand Arrows is what makes them so good. Buffing that and not getting OHKOed by Ice? Sounds sweet. Basically, the best looking Terastal users to me don't have to chose between Super STAB and Defensive Typing advantage because changing to a Mono Type does both.
I'm not going to say Offensive Pokemon don't want to change their type though. Flash Cannon Hydreigon is brought up a lot as a lure and Magnezone is usually going to turn into a Fighting, Fire, or Ground Type. It doesn't need Tera Blast for Fighting if it goes for Iron Defense + Body Press.
Ice Types though, yeah. No question about the defensive ones. Mono Ice wants to change type and most part Ice want to lose a type if they don't want to change both. Avalugg is viable in OU as an Ice type, so imagine it when it can turn into a Steel type! Defensive Pokemon look really interesting because they don't lose much from losing or changing STAB.
I'm not entirely sure about if Tyranitar wants to lose the Rock Type, but becoming Pure Rock for keep Sandstorm's SpD buff is tempting. Flying for resistances to Fighting, Ground, Grass, and Bug looks neat if it if you're okay with the Rock weakness.
Side Note: I can't say Rock/Dark is a low tier type the only Pokemon with that typing is Tyranitar. :P
All that said, keep in mind that not commiting a Pokemon to Teralize lets you have flexability with who you Teralize instead of being comitted to one Pokemon that needs it to be viable.