@ Hidden Power being the primary option; the only thing I fear with this is, without a reliable secondary attack it would be slightly easily walled itself, especially with the likes of HP Ground, Elec, Water and Fire.
The other thing would be the argument of using STAB over SE; if (and this is a big IF) CAP10 is given Multitype, that would turn the HP into STAB, potentially depending on if you treat countering (as switching in) or checking (as in forcing out or killing) as the higher priority.
In regards to Ability, I'd rather opt for two out of many immunity abilities (Levitate, Motor Drive, Flash Fire, Water Absorb, etc.). The whole point of this counter is that it must switch into the designated threat with little to no impunity. Doing this would primarily revolve around rendering the foe helpless against you - if they can't even touch you, how will they expect to kill you?
The typing would also likely center itself on things such as immunities and resistances.
Howeer, as discussed, giving it a set typing does hinder it immensely, as it wouldn't be able to counter another specific threat otherwise. As a result of this, I would think that Multitype seems to be the way forward in order for it to defeat what it has been customized to. And what's more customize-y than changing your typing completely?
The only roadblocks I can see with regards to Multitype would be the lack of leftovers recover, and the fact that Multitype overrides your typing completely, so if CAP10 were a dual-typed Pokémon by nature, then it would become mono-typed upon being given a plate. This is technically a double-edged sword, as you become weak to new things but resistant (and immune, in some cases) to others. That will be a discussion for another time.
However, with Multitype comes the sheer versatility CAP10 requires in order to counter 'virtually anything.' The Trick immunity is an added plus.
The talk of Porygon2-but-better is also a bit of an odd subject for me. I tend to agree with Rising_Dusk on the subject of Trace. Sure, switching into Heatran stops it, depending on your type. Switching into Salamence/Gyarados gives you a little security while it DD's or hits you softer to do...things. However, not everything is good with Trace either - Tyranitar, you're rendered useless against. Rotom - it doesn't care. Similarly with the likes of Latias. Blissey laughs at you. Skarmory moreso. Heracross doesn't pack status anyway. Infernape could probably kill you before your boost activates, and that's if you have a Fire-move at all. Lucario doesn't have flinching moves outside of Bite and Astonish so LOL.
Also I'd rather have Multitype or an Immunity-Ability over Trace any day.
Porygon2's movepool however, is a different story; it has just enough of what it needs to do what it needs. Discharge, Ice Beam, Toxic, Recover, etc.
It doesn't waste time with Taunt, Spikes, Rapid Spin, Roar or any of that rubbish. It comes in and makes your opponent think 'crap.' and that's a mission successful.
So in a similar vein, CAP10 doesn't need a movepool full of support. It just needs a reliable recovery, enough attacks to be able to hit back at whatever it is you're gearing it for.
However, I stand by my point that risking so much to counter a certain threat must have its repercussions if you don't have anything to counter. Okay, if not, you still have a reasonably bulky status-er, but that won't really achieve or help as much as it should, like if it were all-out shutting down one of your team's big threats.
With regards to stats; I'm no numbers whiz (well okay, yes I am) but something with average speed and obviously good bulk is necessary.
Deck has been talking about Base 90 Speed; which I'm agreeing with. +Ve nature and some speed allows it to deal with neutral Base 100's and the like.
Some people are losing sight of what CAP10 is supposed to do; be customized to 'soft counter' just about anything imaginable. The words 'customize' and 'counter' should really be defining how we go about this.