Considering the complete and utter failure of any fairy type not named Xerneas when it comes to counter dragons, I doubt any of them except maybe Hydreigon and Haxorus will drop to UU.
If the fairy type was slapped onto something with stats comparable to the common OU dragons (re: 600+ BST) I could definitely see several of them dropping to UU, but right now the only truly great fairy types in OU are Azumarill who frankly isn't that much of a threat to most dragons (it's slower than all of them and they resist Aqua Jet) and Mawile who is completely useless without its mega evo.
I wouldn't really call it a "complete failure" to be honest; I'd rather call it unfinished. Salamence and Hydreigon especially have taken a big hit now that the two premier fairies (Azumarill and Togekiss, and they're definitely moving up) are more than comfortable short-cutting its sweep, and even the more popular dragons' options
have been limited thanks to the fairies; things like SubSD-Chomp, Multiscale-shuffling Dragonite and mono-attacking Latias are no longer viable.
Countering dragons in the traditional sense is not their best suit, though, because of how skewed the fairies' stats are, and arguably the shift to bulky offence and ubiquitous priority have hurt dragons like Hydreigon and Salamence just as much.
Salamence is more likely to cling on to the lower end of OU for some time I think, but Hydreigon and Haxorus were bound to drop any time now (though they may or may not go the way of Staraptor). The rest of the favourite dragons are still top-notch. Zygarde definitely can hold its own in OU, but because people rather underestimate it, it may not see so much use. (I seriously do not hope Kyurem-B drops down, however unlikely. That way leads utter madness.)
As for other potential UU's, I'm rather on the fence about
Breloom. The huge number of nerfs it got was really bad, worst of all being the Sleep nerf. The fact that OU is drowning in Aegislash, Gengar, and Talonflame really doesn't help either. On the other hand, I'm surprised at how many current threats it checks: Azumarill, Excadrill, Rotom-W, and M-Kangaskhan just to name some. It also still has a very good boosted priority on its side and alternatives to Low Sweep, including Force Palm and its nice paralysis chance. Certainly usable in OU, though like the barely-used OU dragons mentioned above, it might be in danger of dropping soon.