Thick Fat Togekiss: This is an odd idea, it seems weird falvor-wise, but is excellent battle-wise. Losing an Ice weakness and gaining a Fire resist is pretty sweet, and since Serene Grace was used mainly to be cheap with ParaFinch (and since neither of her other abilities helped her), trading abilities is a very minute loss. Her defenses are already pretty sweet, but with a new Fire resist and Ice neutrality, she can take on threats such as both Mega Charizards, Talonflame, Victini, Volcarona, Entei, Mamoswine, and Greninja, much more effectively. Her presence in OU would much more significant with Thick Fat, and would vote for if not for two other options that catch my eye.
Slack Off Rhyperior: This guy has always had awesome physical bulk, and Solid Rock really helped with his godawful. defensive typing, so giving him reliable recovery is a no-brainer. He'll certainly have competition with Hippowdon, who has better special defense, lacks any 4x weaknesses, and has half the number of weaknesses, but Rhyperior has Solid Rock, much more attack, important Fire and Flying resistances, doesn't have an ability that hurts teammates, and much better physical bulk. Other than that, what's already been said about Hippowdon can be said about Rhyperior. An excellent choice that I would love to use.
Dragon/Steel Druddigon: No flavor, not too much viability. Losing Dragon, Ice, and Fairy weaknesses is a godsend to Dragons, but gaining Fighting and Ground weaknesses is quite unfortunate. Dragon/Steel coverage isn't very ideal, since opposing Steel-types can wall this pretty well, and if running STABs, then he has a bit of 4 move-slot syndrome. Without Fire Punch, Scizor, Ferrothorn, and Skarmory fear nothing from him, without Sucker Punch, he has no way of keeping the pressure on opponents, and can't even outrun Blissey of all things, and without Earthquake, Heatran literally has a field day with him. Not the worst theorymon, but far from the best.
Prankster Gourgeist: This guy would be great if he got Prankster, with Will-O-Wisp, Trick, Destiny Bond, and Leech Seed making him quite versatile. But another trait that sets him apart from other Pranksters is having a decent base 100 attack to fight back with, and Seed Bomb and Phantom Force to deal damage and Seed stall, respectively. Though an interesting move to use could be Worry Seed, getting rid of troublesome abilities such as Gale Wings, Technician, Prankster, and Guts, really messing with some opponents dependant on their abilities. Another theorymon that I would love to use.
Right now, I can't decide between Slack Off Rhyperior and Prankster Gourgeist, both of which have their drawbacks and strengths, but I'd be fine using either one.