Right now, I'm in complete support of Volt Absorb. Volt Absorb is a great ability for CAP11, as it lets it come in on many of Togekiss's counters and other Pokemon that it has some problems with--Starmie, Zapdos, Rotom-A, Gengar, Electivire, and Jolteon. All of these Pokemon tend to go for Thunderbolt against Togekiss (well, Starmie may also use Ice Beam though, so I suppose that one's a toss-up, but still), which means with Volt Absorb, CAP11 would gain a free switch-in against them. This gives CAP11 great defensive synergy with Togekiss (especially with the number of Pokemon it helps CAP11 to switch in against), which goes a great deal in helping it to be Togekiss's perfect partner. In fact, as Rising_Dusk said, this is the type of thing that I really would consider to be vital in a perfect partnership, so it really does seem to be the route to go, to me.
Motor Drive gives the same synergy, so I wouldn't mind too much if it was chosen over Volt Absorb, but the speed boost does worry me a little, so Volt Absorb would probably be the safer route here.
As for Trace, it does cover some of the same Pokemon, but as has been brought up, it's totally useless against Pokemon like Zapdos, Rotom-A, Gengar, and Starmie, so Volt Absorb seems to be the better option to me.
Limber has also been brought up, but most of these Pokemon don't tend to use Thunder Wave on Togekiss, instead opting for trying to outright kill the thing with Thunderbolt. Blissey's really the only thing that might try to paralyze Togekiss (and even in Blissey's case, it's just as likely to try wearing down Togekiss with Toxic+Seismic Toss, as it is to paralyze it), so Limber's utility seems really limited to me, and Volt Absorb seems to be the better choice out of the two.
Lastly, one other ability that I've seen get some support is Thick Fat. While it's true that Togekiss does have an ice weakness and this would give CAP11 an ice resistance, which is good for defensive synergy, Togekiss tends to have more problems with Pokemon like Zapdos than it does the Bulky Waters that tend to carry moves like Ice Beam. And as of now, even CAP11 doesn't exactly like coming in on Zapdos's Thunderbolts that much. So, while Thick Fat isn't exactly a bad choice on CAP 11 and would be one I'd be willing to work with, Volt Absorb seems to do a better job of giving CAP 11 defensive synergy with Togekiss.
Now, for the question that keeps popping up: If we give it Volt Absorb, will it become Gyarados's Perfect Partner instead of Togekiss's? To start off with here, I'd really like to point something out: Togekiss's and Gyarados's weaknesses:
Togekiss:
Electric: 2x
Rock: 2x
Ice: 2x
Gyarados:
Electric: 4x
Rock: 2x
Let's also look at their defensive stats for a second:
Togekiss:
85 HP/95 Def/115 SpD
Gyarados:
95 HP/79 Def/100 SpD
What do we see here? Gyarados and Togekiss have two of the exact same weaknesses, with Togekiss having one weakness that Togekiss does not. In addition, both of them are more bulky on the Special side than they are on the Physical.
What does this tell us? Well, with all that being the case, it's only natural that the same attacks that give Togekiss trouble would give Gyarados trouble too. The only thing that really gives Togekiss some trouble that doesn't bother Gyarados that much are Ice attacks. As a result, we might be tempted to focus just on those, as that's something that Togekiss has that Gyardos doesn't.
However, the thing is, CAP11 is supposed to be Togekiss's perfect partner, and should be able to come in on what Togekiss has the most problems with. And as I mentioned in the Thick Fat section, Togekiss has a whole lot more trouble with Pokemon like Zapdos than it does Bulky Waters. In order to be successful, CAP11 has to be able to help Togekiss with Pokemon like that the most that it can.
But if we do that, what about Gyarados? Again, this thing has to help Togekiss with its weak spots. It just happens that Gyarados is week to the same attacks as Togekiss. However, we can't let fear of letting this thing become Gyarado's perfect partner drive this project off-course. This thing is intended to be Togekiss's perfect partner, and Togekiss has problems with the same attacks as Gyarados. Therefore, if we let fear of CAP11 being too much of a Gyarados partner drive this CAP, and don't let it come in on anything that bothers Gyarados, that means that CAP 11 also won't be able to come in on anything that bothers Togekiss. And if that's the case, this project will naturally fail, and if at all possible, we should of course avoid that happening. Thus, we just have to accept that the same type of attacks that bother Togekiss bother Gyarados and due to this fact, the same defensive synergy that's needed to make this project succeed also benefits Gyarados.
However, this leads to an interesting point: Togekiss and Gyarados ae bothered by the same attacks, but by different users of them, and have different counters. In other words, yes, it's true that neither Pokemon likes Thunderbolts and both would like a partner that lets them come in for free, but the two of them would like this partner to come in on different Pokemon. Gyarados is bothered most by Pokemon like Vaporeon and Suicune, which don't quite bother Togekiss as much. This is the key factor, which allows us to give CAP11 Volt Absorb, while not turning it into Gyarados's perfect partner instead: by focusing on the Pokemon that give Togekiss the most trouble, like Rotom-A and Zapdos, instead of ones like Suicune and Vaporeon, which are very troubling to Gyarados.
As an addition to this, there's already a Pokemon with such an ability that people have tried to partner with Gyarados: Electivire. In addition to having Motor Drive, Electivire is also an Electric-type. Yet even so, the GyaraVire combo has rather... mixed results, which is why it's not exactly the most common partnership seen, especially amongst more skilled players. This is because that while Electivire draws in the T-bolts & such that Gyarados hates and gets a nice speed boost of it, the partnership is really slanted towards Electivire's end, with it getting the bulk of the benefit. In addition, even with the speed boost, it still has to either use low base-power moves on the Physical side or otherwise use its hardly-terrible-but-not-exactly-anything-to-brag-about Special Attacking stat.
In any case though, the fact that the GyaraVire common isn't seen more due to factors such as the partnership really being too heavy towards Electivire seems to give credibility to the idea that as long as we focus on Togekiss's counters and not Gyarados's, things should turn out just fine. And since CAP11 is already Fighting/Dark, instead of pure something like pure Electric, like Electivire, I believe that we're already on the right track here, so this shouldn't be too much of a problem.
tl;dr--For all the reasons above, I believe that Volt Absorb is the best choice of ability for CAP11 and don't feel that there will really be any problems with it making CAP11 too much of a Gyarados-partner instead (and even if it did, I feel that to be irrelevant, and something that we'll just have to deal with, as Togekiss and Gyarados are weak to many of the same attacks, and what benefits Togekiss defensively will many times also benefit Gyarados. Thus, conversely, if we ignore Gyarados's weaknesses, we'd also be ignoring Togekiss's, and if we do that, the project will naturally be a failure. As a result, even if something like the ability also greatly helps Gyarados, that's something that we have to deal with and hope won't be a problem, as otherwise we'd be hurting CAP11's ability to be a perfect Togekiss partner just as much, if not more so).