Almost Any Ability: Surf-spam with Storm Drain team, Lava Plume with Flash Fire team (I guess), even more Discharge-abuse teams, completely random Swift Swimmers/Sand Rushers/Chlorophyl, arbitrary choice of weather setter (no more suffering through Politoed and Ninetails!), much more difficulty overall telling whether any given team is a rain abuse team or not (Protean means ANY Pokemon can STAB-abuse weather so long as it as Water/Thunder/Hurricane/Weather Ball [Actually, what happens when Protean meets Weather Ball-in-weather anyway?] or Fire/Solarbeam/Weather Ball/Growth/Synthesis etc), Earthquake spam abusing Levitate on everything, Aerilated Boomburst being combined with Telepathy on anything you like, Petal Blizzard alongside any Pokemon you care to name (Because you gave it Sap Sipper).... other stuff I might say later...
Metagamiate: The only thing I have to add that hasn't been said is the very silly, dumb thing of having Water Absorb Lanturn Soak Noivern. I doubt it would be in any way practical though.
STABmons: Suddenly every random Rock type has Wide Guard and every random Fighting type has Quick Guard and Mat Block. Every Fairy Type has Crafty Shield (Screw all you Pranksters), Rage Powder on every random Bug type, Follow Me on every random Normal type (And the Eeveelutions, and, not that anybody cares, on Talonflame, among others), Tailwind on every Flying type (Not many additions there) and Trick Room on every Psychic type (Most of them don't care), Powder on every random Bug type (Yeah it's in Singles too, but it would be a lot more relevant blocking a Heat Wave while your buddy is free to work), Land's Wrath on every Ground type (Who needs immunity to Earthquake?), and probably bunches of other things I'm not thinking of specifically relevant to Doubles.