With Dexit wiping out nearly all of last gen's UU staples (and likely moving those that stayed up to OU), I'd expect this generation to be pretty unlike USUM. It'll feel pretty empty for me for a long time, but at the same time it'll be interesting seeing such a stark change due to the sheer lack of available mons compared to gen 7. I'm interested in seeing which old Pokemon will make a splash up in UU that were too outperformed earlier on, though I'd also like to touch on some of the new Pokemon that may join us this generation:
I think
Duraludon has some potential as a tanky, powerful rocks setter. Dragon/Steel offers a TON of useful resistances and I'd expect the general shortage of either Fighting- or Ground-types to be appreciated as well. Its high Special Attack and solid STAB combination could definitely be promising alongside its great physical bulk, though it also has a pretty shallow coverage movepool and could definitely be hampered by its low special bulk. Regardless, I think it'll offer a lot of role compression between its offensive prowess, access to rocks, and ability to check some mons I could definitely see ending up in UU, like Indeedee, Heliolisk, and Escavalier. Its overall viability may rely on the structure of the meta, but I think it'll have at least something of a niche.
Polteaguist is a Pokemon that I think could easily become a UU mainstay. It has access to Shell Smash and a HUGE Special Attack stat that I think will both serve it well in being a great late-game cleaner. Its access to Stored Power further boosts its potency and will likely allow it to bust through stall teams, though its inability to dent Dark-types really at all is concerning. I'd imagine Will-O-Wisp presenting itself as a decent option to at least cripple some Dark-types we may come across, like Weavile, though I could also see it pulling off some shenanigans with Strength Sap or Self-Destruct in its fourth slot. Dark Pulse could also be an option if Indeedee ends up being alright. This thing's poor defense and iffy Speed could definitely make it less promising than I'd hope, but I still bet it sees some use given its tools.
Speaking of
Indeedee, I think the more offensive male variant could shine in the tier thanks to its access to Psychic Surge. With Choice Scarf I could see it shredding some offenses, and the general lack of super fast mons makes 95 Speed seem more passable for sets like Choice Specs and maybe Calm Mind as well. Its sorta average stats do worry me a bit, though I think it could pull through under the right conditions, and even aside from its offensive presence Psychic Terrain is great support. With Shadow Ball, Dazzling Gleam, and Mystical Fire it should have workable coverage, and no more Pursuit means that every Psychic- and Ghost-type has a much easier time sticking around throughout any given match.
Inteleon is interesting to me. Its movepool is pretty shallow but it has nasty Special Attack and Speed stats. With Choice Specs I could see it doubling as a solid early- to mid-game breaker and late-game cleaner. Ice Beam and maybe Dark Pulse should give it decent coverage, and U-turn is never a bad move to have access to, especially with its Speed. I think it'll have a solid niche in the tier, though it'll definitely be held back by its inability to break waters (maybe this won't be too big an issue should Jellicent become one of the better choices?). Its defensive utility may also make it a little hard to justify in some scenarios, though it'll be fun to toy with.
Galarian Weezing is admittedly a Pokemon I have mixed feelings on. Defog is pretty limited in its distribution for probably a while yet, and checking some potentially good Pokemon in Sylveon, Mimikyu, and Flygon should serve it well. Unfortunately I think it'll be hard for it to really capitalize on that 4x resistance to Fighting, since I'd expect some of the tier's better Fighting-types to be Lucario, Toxicroak, and Bewear, who all seem decently equipped to break past it. Galarian Rapidash could also be a solid Pokemon in the tier, which would further complicate Weezing's ability to check some Pokemon its typing should allow it to.
In addition to those, I'd expect some Pokemon that may not have shined this past generation to make some names for themselves again in UU this time around. Sorry for the long post, and I'll try keeping this portion short, but here are my opinions on some older mons I think could be potent:
Without Scizor around to give these two massive opportunity cost, I think they should have ample opportunity to shine in this gen's UU. Escavalier excites me SO much. It was such a fun tank in ORAS and should have excellent offensive and defensive potency, especially given how cut down access to Knock Off has become. I think between a tank set with Leftovers, Choice Band, and maybe Assault Vest and SD RestTalk it could have a decent amount of options at its disposal and be a very versatile and important threat overall. Durant could also be a solid presence, especially given its access to First Impression, though I'll reserve excitement for how broken dynamaxing sets could be because BAN DYNAMAX.
I'd expect these guys to become the go-to Fighting-types of the metagame. Lucario is already pretty great in USUM but losing its biggest competitors in Infernape, Cobalion, and Terrakion should only decrease its opportunity cost and make it a real household name, especially considering its slightly shabby Speed may not be as large a problem. Toxicroak has some potential in a metagame that seems it could favor the likes of Sylveon, Milotic, and Inteleon. It could run into some issues but again the lack of competition should certainly help. Bewear has pretty consistently hovered the gen 7 vr's lower B area, largely due to competition, so I think the higher demand for Fighting-types should certainly help, as well as the possible presences of some Pokemon it could check quite convincingly, like Escav and Weavile.
Lumping these Fire-types together since, like a lot of the other stuff here, I think their lack of competition compared to gen 7 should help them a lot. Ninetales could prove a solid wallbreaker with Drought + Nasty Plot, while I'd also imagine the lack of Drought abusers available should prevent the ability from getting banned. Salazzle will benefit a fair deal from Infernape being gone, though I also think it can capitalize on a lot of potentially fearsome threats in the meta, like the aforementioned Lucario, Toxicroak, Escav, and Durant. Both of them lack the defensive utility Infernape's typing at least yielded it, though we'll just have to wait and see how the metagame rewards their specific traits.
Sorry this post was so long! I have even more thoughts swirling around but figured they'd have to wait for another essay LOL. Thanks for bearing with this post if you made it all the way through

I'll be scared to enter a gen without my precious Infernape but I'm intrigued to see if any of the predictions we've seen in the thread will come true. Let's hope for a fun gen regardless of some of our early preconceptions!
Also free Heliolisk