Overall, I am happy with the way this thread has been going. Rather than most CAPs, which are more than anything an exercise in yelling at people to exercise restraint, we've already restrained ourselves simply by choosing Belly Drum, and it's now an incredible puzzle to figure out the proper answer. We're working with a real catch-22 this CAP: "A Pokemon that can't set up and sweep is too weak and a failure. But if it can set up and sweep, then it's too powerful and will be a failure." I'd like to apologize to everyone, then, for the way I worded the OP, because I kind of made it sound like we were choosing either to be able to set up or to be able to sweep perfectly, and the other one would be basically neglected. Taking such an approach is not really plausible with any sweeper: either its sweeping conditions are too specific and it is impossible to set up, and put aside for something more consistent, or its strength will be lacking and it'll be put aside for something that actually accomplishes things. On the other hand, taking an approach which perfectly accomplishes both of these things (or even one) is going to be overpowered—i would like us to both be able to set up imperfectly and to sweep imperfectly. This said, I'll address some points, and then talk about the major abilities that have gained ground:
Nyktos said:
This is another excellent choice if aiding setup is deemed the best use of our primary ability (though it isn't).
(Yes it is). One turn of set up is always better than two. Apart from our ability, ways to avoid taking things like Leech Seed and Thunder Wave are basically... Taunt, Sub, or force a switch. I shouldn't need to point out why Substitute is not a great idea when we're already losing 63% hp between SR and BD; forcing a switch against physical walls at +0 sounds like a bad idea considering we want to be stopped by physical walls at +6, and Taunt leaves us very vulnerable with an extra turn at +0 with nothing to help us sponge hits. Once our CAP is at +6, it can solve any problem by just doing damage (not that we want it to be able to solve every problem but it theoretically could), without the aid of an ability. However, Ability is the only place that we can really help the problems that plague us at +0.
Motor Drive:
The approach which Motor Drive takes is one I am very fond of in theory. Rather than taking an ability which aids either setup or sweeping, and then relying on Movepool or Stats to pick up the slack, Motor Drive takes the 'why not both?' idea right from the bat. The main question which plagues me with this ability is: will it be enough? The most common Thunder Wave users—Celebi and Ferrothorn—each have another option for a support move which would cripple CAP6, the former using Perish Song and the latter using Leech Seed. On the same token, Motor Drive doesn't really let us switch into any offensive Electrics other than Jolt—I fear hydro pump or wow from rotom-w and focus blast from thund-t, and we can't scout choiced electric moves because they're probably Volt Switch. As such, it'll be extremely difficult to actually get the motor drive boost, as Electivire has shown. Motor Drive does afford us two setup opportunities—on Blissey and Forretress—and immunity to one revenge killer—jolteon (and Magnezone without hp fire)—but is that enough? Without a consistent promise of +1 spe from an elec move, I'm not too convinced that it will actually help enough.
Magic Bounce:
The exact opposite of what I just said about Motor Drive, I fear this ability would help
too much. Lots of people have been pointing out that a MB CAP would neglect belly drum in favor of a defensive set. Even ignoring the argument that the CAP would neglect belly drum (i mean if espeon's psychic typing and 65/60 defense can use a magic bounce support set, what can't), I see another problem. We can't have Magic Bounce let us set up on what we want to set up on without also throwing the entire concept of a defensive answer to CAP6 out the window. The same ability that reflects Hippowdon's Whirlwind reflects Skarmory's too. Defensive checks, such as Skarmory, Heatran, and Jirachi could still exist, but they'd all have to pack a way to KO the cap, instead of using their usual methods. And until the CAP is down, defensive teams can't really risk the usual stall setup. I understand that a Magic Bounce CAP could still have a healthy amount of offensive checks and have a solid presence in the metagame without being underpowered or overpowered. This is more than I can say for any of the other abilities I have yet seen proposed. However, even if we can keep Belly Drum as the main set, a magic bounce belly drummer would be
incredibly difficult to handle for defensive teams. while this isn't a necessary violation of the concept, it's something I, like capefeather, would rather avoid.
Unburden:
While this ability does give the CAP more breathing room after having set up, it still doesn't help us set up. Unfortunately, a Pokemon that can't set up, no matter how good it is after setting up, is garbage. I don't like Unburden, because it does like I said in the top of the post: focuses on sweeping perfectly, while letting setting up fall by the wayside.
Any Ability that lets us blow through our counters but doesn't help us set up or avoid being revenge killed (like Mold Breaker):
because +6 wasn't enough power, and 50% hp isn't low enough?
Regenerator:
The above poster has made a good case for Regenerator until you consider that the argument is basically that Regnerator CAP won't use Belly Drum, which goes against the concept itself. When it comes to using Regenerator because then the CAP gets multiple opportunities at setting up—it's a fair point, but why should a Pokemon that can't set up the first time be able to set up any more often afterwards?
Intimidate:
I really do like Intimidate. It, like Magic Bounce, allows us to set up on a wide swath of Pokemon, but it, unlike magic bounce, is not horribly overpowered and threatening to break an entire archetype across its knee. At first glance, our typing really works against us, as almost all physical attackers carry neutral Rock or Fighting coverage, which is still enough to break us after Drum and Rocks. however, intimidate turns a lot of those "just enough"s into "not enough"s. I would encourage everyone to pay more attention to intimidate.