It might still be early, but as time passes and we slowly get acquainted with how OU is playing out, it starts to make a bit more sense to speculate what will rise and what will fall.
Which former UU mon will rise to OU?
We're losing our two best fairies. I'm seeing Azumarill and Togekiss a lot in OU. Even though I haven't used Azumarill that much in UU, I'm gonna miss it. Togekiss, not so much.
I haven't seen too many Blastoise, but that's a legit OU spinner. In Gen 5, we were all complaining about how spinners lose to blockers, and how Blastoise has to spend 2 turns just to spin (best case scenario for Blastoise is Cofag gets to +2, Shadow Balls and gets roared away). Well now instead of Foresight, Blastoise can use Dark Pulse off Mega Launcher boosted 135 SpAtk. It's OU worthy, I kinda hope that all those other megas push him down to UU. He'll be a force, but hopefully he'll be balanced by all the powerful OU drops
Of the fire types, I see Rotom-H as most likely to rise up. It's a great counter to Talonflame, even resisting U-Turn. Rotom-W is the only thing keeping Rotom-H down, but Heat can absorb WoW and can hit grass types like Ferrothorn. Rotom-W is more useful than -H but Rotom-H does have its perks.
I've heard people talking about Crobat, but I haven't seen it very much.
Roserade is interesting. Its defense buff is probably not a selling point, but it does help it out in UU. The changes to breeding mechanics makes Rose more versatile. Offensive poison is a bit better than before. As OU-worthy as I think this is, ultimately I think it's staying in UU.
Fall of the dragons
The two dragons, Flygon and Kingdra, were major forces in UU. UU struggled to keep up with Flygon's CB Outrages off of 100 Attack and Kingdra's DD or Manual Rain Sp Attacks off of 95 offensive stats. What happens when 147 Attack worth of Haxorus (who was on the verge of falling at the tail end of Gen 5) inevitably falls to UU?
Dragons will inevitably fall down for two reasons. First, fairies are supposed to put dragons in their place. More fairies means less incentive to use dragons, means dragon usage should go down. However, I think a bigger reason is simply oversaturation of dragons in OU. OU was so dragon dominant that a 147 attack dragon dancer was considered bad. Gen 6 gave us some more dragons, and while they all play differently, you're typically not gonna have 3+ dragons on your team. There are just too many dragons to fit on people's team, so SOMETHING is bound to drop.
At the end of last gen, Hydreigon, Haxorus and Kyurem-B (am I seeing this right?!?!) were on the verge of falling. Gen 6 added Dragalge, Zygarde, Noivern, Goodra Tyrantrum and a few megas (Ampharos, as Charizard is probably going to OU. I mean... it's Charizard, who cares if you don't feel it's OU worthy, people are gonna use it). Dragalge will almost certainly drop. I see Hydreigon and Haxorus (and hopefully not Kyurem-B) getting pushed down. In UU, they'll have a reason to run speed nature. The rest of them are all good, though I see Tyrantrum dropping. Zygarde is absolutely OU worthy, though I see it dropping because people will be attracted to DD and will think it's just a bad Dragonite.
Depending on what falls into our lap, Gen 6 UU may be the tier for dragon spam.
Who will stand up to the dragons?
(more to come when I get more energy. I wanted to discuss Metagross, UU fairies, and certain megas. Additionally I'm wondering about how Staraptor and Froslass would do)