Only 99 new mons added this gen (including alolan forms), probably the least expansive out of all the transitions. I don't appreciate some of the nerfs, either (they nerfed everything that is good about paralysis and did not address the main issue (FPs); Gale Wings nerf (rip talonflame)).
The legendaries are going to shake the meta. Test Bot covered the Tapus, but I am particularly excited for Bulu. Tapu Bulu would be a saving grace to all the grounded mons susceptible to Landotrus-T's Earthquake. Tapu Bulu itself can pretty much switch into Landorus-T's moves with impunity, as it essentially quad resists EQ, resists Superpower & Knock Off, and only take neutral damage to U-turn and Rock Slide. Will Sludge Bomb Lando-T become a thing? Who knows. It's also a nice check to Azumarill, being faster and resisting Aqua Jet + Knock Off. With Grass Terrain boost, Wood Hammer is effectively a Grass-type V-create with recoil instead of stat drops! Even after an Intimidate, Adamant Bulu's Wood Hammer is scraping off 70% min from Landorus-T, and it inflicts 38% min to Kyurem-B lmao. It also has good coverage to address Fire-types with Stone Edge, Rock Slide, and Superpower. Koko has an amazing Speed tier with a free boost in its Electric STAB. It also possesses Brave Bird; with LO it can easily 2HKO Amoonguss with Brave Bird :d
Xurktree, Buzzwole, & Kartana seem to me the most formidable Ultra Beasts. Insane offense with Beast Mode to further ramp them up in power with each KO x_x. Worth noting that Kartana is a solid check to Mega Diancie, although I don't know how well it would take Earth Powers and Moonblasts with its abysmal 59 HP / 31 SpD.
Marshadow has the golden Ghost + Fighting combo with an actually stellar physical Ghost move with excellent movepool coverage. PLUS Technician Shadow Sneak xd. Magearna also have a strong Fairy/Steel typing with great stat distribution and amazing movepool (Trick Room, Fleur Cannon) and an ability that is Beast Mode on steroids.
Outside of the legendaries, it's hard for me to predict which new mons would actually stand out. There are quite a few Wide Guard users iirc, so we don't have to always rely on Aegislash for spread protection any more. Incineroar has Fake Out + Intimidate, so it may compete with Scrafty as a TR supporter, although Scrafty is a lot bulkier and isn't weak to Quake Slide. Alolan Raichu may be a powerful cleaner late game with Electric Terrain doubling its Speed and boosting its STAB, only susceptible to Shadow Sneaks & Sucker Punches. Vikavolt is better than Xurkitree as a TR sweeper (lower Speed), but otherwise Xurkitree is better overall. The only advantage is Vikavolt's EQ immunity thanks to levitate. Belly Drum Kommo-O is also intriguing... can get a free speed boost with Adrenaline Orb, too, if Lando-T ever show its ugly face.
I don't think the new weather users would make weather more prevalent. The pre-existing weather users are arguably better than the new ones (although Pelipper may usurp Politoed as a Rain inducer). We may see more Terrains than weather as field condition, imo.