Oh dear, Togekiss would probably be really broken in UU, speaking as someone who has played a lot of BW UU, I will say that Togekiss was really strong last gen and pretty close to being broken back then. Seriously this thing is a bulky mon and almost impossible to OHKO, and it provides ridiculous support with T-Wave, and paraflinch is cheap and any paralyzed mon basically has like a 5% chance to win. The NP stallbreaker set crushes defensive mons and Air Slash is cheap as always. Its new Fairy-typing means it can basically laugh at Hydreigon, Krookodile, Mega Absol, Mienshao, and a horde of other mons. And again, this thing is surprisingly really bulky (although it's kind of fat so I guess it's not surprising). This thing will be really strong, and in my opinion possibly broken, I think it won't last very long in the tier.
Diancie...well, it will be interesting. I guess it's a nice Hydreigon answer as well, which we kind of need, and its typing isn't shabby defensively. Diamond Storm and Moonblast give it good offensive presence, and it would be a decent bulky Pokemon, and it should do okay I guess as a Mega Absol and Hydra counter. That said, it doesn't have that many useful resists, and Krookodile has EQ so yeah.
The other drops don't seem that interesting. Vaporeon, Quagsire, and Smeargle were all here before, and they were alright. Vaporeon was an okay Wish passer, it'll be the same as before, except with competition from Alomomola: having Roar and offensive presence is nice though. Quagsire will have a niche in Unaware but that's basically it, and Smeargle is an okay lead I guess lol, though I used it quite a lot before it moved up. Espeon might drop but it will still be ass lol, I think it belongs in NU but whatever.
Regardless, as said, the move-ups will be more interesting, we're bound to lose a lot of meta defining mons like Slowbro and Heracross, which will really shake up the metagame, as those two were significant mons in the meta. Mew moving up would be interesting as well. The meta will definitely shake up with some top threats leaving and a new possible top threat joining.
Just some thoughts.