I'd like to address how I believe the priorities should be set for this Pokémon. Before I do this, I'd like to distinguish between a
technically uncounterable Pokémon/partnership and a
significantly uncounterable Pokémon/group. The former has checks that are so reliable that they might as well be counters most of the time. This happens because, to beat the prospective check, the Pokémon/group has to run an entirely different set, or replace a move with an inferior one as far as beating multiple more popular threats is concerned. The latter has checks that aren't so reliable because the Pokémon/group doesn't have to bend over backwards and vary more than maybe one approximately equally valid move to beat each check. Essentially, this is the difference between top OUs like Gengar, Infernape and Lucario, and the recently banned suspects Lati@s, Salamence, Garchomp, etc.
Now for the priorities:
1.
The partnership should be significantly uncounterable by one Pokémon. I'm basically echoing bugmaniacbob here, with the "this partnership will have checks regardless" argument, but I do believe that this is vital for the pairing's success. People often look at counters to defensive cores and decide that this pairing should have a counter as well, but defensive cores can beat their counters with prediction regardless, while an offensive core like this one doesn't have such a luxury. To have two Pokémon beaten by one just feels wrong for a combo that's supposed to be delivering a similar return to the opponent.
2.
The partnership should be technically uncounterable by a rival pairing. Let's face it. This pairing will have many rivals in OU. Between SkarmBliss, CeleTran and GyaraJolt the only way for this pairing to fit is if it can give all of them something to think hard about. If a certain pairing beats this one reliably, then people will just spam that pairing and win. But I don't think that beating all rival pairing should be an easy task. We don't want this pairing to be broken, now!
3.
Togekiss should do for CAP 11 what CAP 11 does for Togekiss. By this, I mean the following:
Fuzznip said:
I really want to focus this CAP in the direction of being offensive. I want something that may not be able to come in directly on all of Togekiss's weaknesses, but once in, it can devastate the things that Togekiss hates and put the opponent in a rough situation; Should they sacrifice what they have that can beat Togekiss or sacrifice something else on their team?
It would be great if Togekiss could do the same thing to CAP 11's counters. The solution to this would be to reduce the ability of some of CAP 11's prospective checks to check it. If a CAP 11 counter is just going to bolt when Togekiss comes in, it shouldn't be a counter.
4.
The partnership of Togekiss + CAP 11 should be decidedly superior to partnerships of Togekiss + different Pokémon or different Pokémon + CAP 11. Of course, this refers to all the warnings about pairing CAP 11 with Gyarados, Zapdos, Rotom-A, etc. I've put this lowest because I truly believe that the other three are more pressing and more doable. If we can't achieve this without compromising on the other three, that's too bad, I suppose, but I think that we can achieve this.
I'll be assuming that the common set for CAP 11 will be the following:
CAP 11 @ Life Orb
4 Atk / 252 SpA / 252 Spe Hasty/Naive
Aura Sphere
Dark Pulse
Close Combat
Taunt
In addition, reachzero seems to have done the sweeping assessment that I would have done, so I'll trust his judgment instead of doing it myself only to result in an assessment that's a repeat or an inferior job.
This is already taking a long time to post, so I'll leave it here for now and maybe edit this post with specific examples later.