One thing I might explore with this metagame is the use of Dry Skin and Poison Heal on offensive setup sweepers. Both obviously have their advantages: Poison Heal makes you status-immune and works regardless of weather, while Dry Skin stacks with Leftovers recovery and makes you Water-immune. However, both essentially let a Pokemon recover impressive amounts of health each turn. One Pokemon that could be pretty interesting is Dry Skin Terrakion with a SubSD set. I'm theorymonning right now, but I'd imagine that it would work like a souped-up Toxicroak due to its better power, speed, bulk, and STAB coverage. In addition, Terrakion would lose its Water weakness in exchange for a less effective Fire resistance. Even more dangerous might be SubCM Keldeo. While it doesn't appreciate the Water immunity quite as much as Terrakion, it does abuse Rain much more effectively with its STAB Surf. As far as Poison Heal goes, I could definitely see some faster defensive Pokemon taking advantage of it very well. For example, CM Latias could run Substitute + Protect to stall out dangerous moves while boosting until its Dragon Pulse is strong enough to ravage the opposing team.
Another neat ability that could be useful on a number of Pokemon is Regenerator. You have many good defensive Pokemon such as Forretress and Ferrothorn that make great switch-ins to a number of offensive Pokemon, but they don't exactly have the most reliable recovery in the world. Regenerator would let them pivot in and out of attacks while staying in shape very well. Cresselia would also love Regenerator, seeing how her best recovery move is only reliable if you can keep Sun up. With Regenerator covering Cresselia's health, that would even free up a moveslot so that you no longer have to pick 2 of Ice Beam, HP Fire, and Psychic STAB, or you could even use her as a status platform of sorts and run both Thunder Wave and Toxic. Hey, if it gave Amoonguss a good niche in OU, I'm sure it could do wonders for Pokemon such as these.
On the offensive side of things, Sheer Force looks to be a monster of an ability. Imagine Sheer Force Tornadus, hitting pretty much everything somewhat hard with its STAB Hurricane and Focus Blast for coverage. Only a small handful of OU Pokemon can avoid the 2HKO from either Hurricane or Focus Blast with Stealth Rock down, with Jirachi being the best switch-in. Even Blissey and Chansey need SpD investment to do so. Download could be insanely powerful as well. For example, CB Terrakion could come in on a host of OU Pokemon and nail an Atk boost, giving it +1 Atk further boosted by the Choice Band. To put that power into perspective, even 252/252+ Landorus-T is 2HKO'd by Stone Edge 61.33% of the time without Stealth Rock. Your best shot at taking on Terrakion would be Golurk, who can actually avoid the 2HKO from Stone Edge after Stealth Rock with just a little more Def investment than the support set on its OU analysis. That is, unless Terrakion starts running Earthquake on its CB sets, in which case Golurk is easily 2HKO'd (that also ends any Nidoqueen shenanigans you might have planned).
Of course, the trick with some of these abilities would be finding Pokemon to swap with. Porygon-Z and Porygon2 are the only Pokemon that get Download, and I'm not sure they'd have much use for Terrakion's Justified. Same with the Dry Skin users alongside Keldeo or Terrakion. Since Prankster is banned from swapping, you'd have to give Tornadus's Defiant to something in order to give it Sheer Force, although it would be nice on a physical sweeper in order to turn Intimidate from a hindrance to a blessing (I'm sure Landorus-I would appreciate this a bit). Regenerator would definitely be a tricky one. Iron Barbs Amoonguss actually sounds interesting, and at least it gets Spore. Definitely lots of possibilities.