Hey. I'm not deeply involved in the CAP metagame (yet), but I'd like to add my two cents about the main typing.
I strongly support Electric and Poison as the primary type, with my vote going to Electric. To me, when you think of creating a Pokemon that's designed to come in and successfully take a hit from a number of threats, you need to start out with a typing that has very few weaknesses, and then later build around that typing to increase resistances (through abilities, secondary typing, and whatnot). While it's nice to immediately come in and start thinking of types that offer a lot of resistances (Steel and Water, for instance), I believe that we should start talking about resistances once we have a general idea on what our Pokemon should be resisting to counter its main typing to make it successful. That is, we should decide on what the main typing is first, then decide what we need to add to make the resistances work. Electric and Poison offer the greatest neutral coverage defensively, with Electric having one weakness and a small amount of resistances, and Poision having two weaknesses (albeit one is uncommon as an attack).
Electric has one weakness to Ground, obviously, and a small slew of resistances to thinks like Steel, Electric, and Flying. That's a really good starting point for a defensively-minded Pokemon. While it only has two useful resistances, we can tack on more during the secondary typing and ability sessions. Electric is resisted by Dragon (albeit most have a secondary Flying or Water type, though), Electric, and Grass, and Ground is immune. That's pretty decent neutral coverage offensively as well, and fifteen of the top 20 threats of the OU metagame take neutral or better damage from Elecric attacks (16 if Latias is voted Uber, assuming the 21st takes neutral). While not as important as its defenses, it does allow it to have some sort of attack to support itself instead of it relying on non-STAB moves to deal damage across a broad spectrum (like Swampert's lack of a Water attack on its most used set). Access to Thunderwave (as most Electric types do) might also help it reduce the offensive threats in the metagame that it can potentially counter. While Earthquake is one of the, if not the, most common moves in the metagame, this can easily be alleviated in the next few rounds of discussion. And, while Earthquake is common, its also one of the easiest moves to counter with a whole team through switching, which is a valid point considering this Pokemon seems to be designed to do multiple switching throughout the battle. Also realize that access to Electric STAB will give this Pokemon a chance at beating traditional Stall teams who run Bulky Waters like Suicune and Skarmory. Remember that this Pokemon will balance the metagame by countering all kinds of threats, both offensively and defensively. I feel that the Electric typing is a good mold to start with. It's something new (very rarely is a Defensive Electric type used save for Zapdos, who is one-dimensional defensively) and unexpected to the metagame itself. I think starting with Electric will be a lot of fun.
Poison gets my second nod, and its much in the same vein as Electric. Two weaknesses in Ground and Psychic, with one rarely being used. Resistances to Grass, Fighting, and Poison. While that might not sound stellar, the Poison typing gets the bonus of absorbing Toxic and Toxic Spikes laid out, which is huge for Defensive Pokemon. The only reason this is not as great as Electric is the fact that its Poison-based movepool won't exactly have good coverage at all, and there are few good attack choices for it except Cross Poison on things like Drapion who can abuse the Crits. You can argue Toxic, but remember that a lot of Pokemon who aren't even close to Poison Pokemon have access to Toxic (Articuno? Empoleon? Electivire?), so the Poison typing is not necessary for that point. I would rather see this as the secondary typing for what's chosen already.
Normal, Fighting, and Ghost typing all are good choices for neutrality as well. However, I see one major flaw: today's metagame is designed to counter Ghosts and Fighting types with opposing Ghost and resisting types, with almost every team carrying a Ghost to counter Fighting attacks, a Steel to resist Normal and Ghost, and things like Pursuit Tyranitar for Ghost and weaker Normals. To me, since these typings are played against each other in such a way already, I believe that it might hinder this Pokemon's usage after the initial switch. It will then become either set-up bait or Pursuit weak (in the case of Ghost), which ruins the point of this Pokemon. A Secondary typing can alleviate this, but even so, I have almost no doubt in my mind that this Pokemon will simply fall into the same rhythm of "switch in to counter, switch out because of counter" kind of strategy because of the roles that Ghosts/Fighting/Normal types play. I believe we're aiming for a Pokemon that is not initially countered by any one Pokemon, but countered by all Pokemon depending on its moveset. This makes it unpredictable, but effective. Getting rid of this kind of rhythm is essential for this Pokemon to succeed.
Now, the same can be said for Electric with Ground types and Poison with Steel types, but here's the difference: because we are preparing to alleviate certain weaknesses of the initial typing, these typings have the ability to use an ability or a move (Levitate, Magnet Rise, etc.) to help this. I personally believe it's harder to find better pairings with the Normal/Fighting/Dark/Ghost/Psychic sets, and they don't really have moves or abilities that help their weaknesses from the start. Look at Ghost/Normal: sure, it gets rid of two weaknesses and keeps only one in Dark, but offensively, Normal and Ghost are polar opposites, as there is no real Normal Special Attack outside of Hyper Beam, and no real Ghost Physical Attack outside of Shadow Claw. Furthermore, it would fall prey to Steel types easily, who will be barely affected by anything thrown at it looking at the type pairing. No Toxic, no nothing. Steel is also one of the major types we're trying to counter, with things like Scizor, Forretress, Jirachi, Skarmory, and Empoleon in the OU environment. Now, look at something like Electric: let's make an Electric/Poison type. Boom: we can give it access to Levitate, thus giving it quite a few resistances and only one weakness. How about Electric/Grass? It removes the Ground and Flying weaknesses and trades it for two 2x weaknesses to Fire and Bug. You now have also given it a way to hit Ground types for STAB. See what I mean? The fact of the matter is, the Ghost/Fighting metagame is way too common, and we don't want this Poke to fall under the same kind of flow.
Water is always a good choice, but I find that a lot of teams now have easy counters to such "bulky Waters" nowadays, like Latias, Vaporeon, Blissey, Zapdos, Jolteon, etc. I just can't shake he feeling that we're just going to see another same old Bulky Water emerge. Whether this is bad or not is the question, but I personally am looking for a creative edge. The same goes for Steel: will it just be used for its resistances? A lot of things are built to ruin Steels too, like Infernape and Magnezone. I just don't want this Poke to fall for the same tactics in the current metagame, y'know? I wouldn't be mad, and its still good typing for resistances. I just think we would potentially see the same things we already see in the metagame with this typing.
As for abilities (to respond to the other discussion in the thread culminating), Levitate is always the best way at removing entry hazards (huge for switching purposes) and alleviating Ground weaknesses. However, I fear that may be too broken, as it automatically gives the Poke an instant immunity to its only weakness AND removes the point of Spikes. Another possible choice could be something like Filter or Solid Rock. This would help reduce the need for neutrality by a small margin, and promote a wide range of possible Pokemon this guy can counter safely. However, I'll wait until we're at the Abilities section to discuss this further.
