I'd like to throw out my support for Bug/Psychic and Bug/Dragon. I was initially against Dragon, but birkal makes a very compelling argument, honestly.
However, one possibility that I don't see raised yet is Bug/Ghost. Of the many things we could do for CAP4, I think this is the most proper route, and here's why:
Passive damage weakness – Weak to SR, Spikes and T-Spikes, Sandstorm and Hail, Poison, Burn, and also Pursuit
Good overall STAB combo – Bug and Ghost together hit all mono-types bar Steel at least neutrally, and moreover, Bug STAB is SE against Dark types that'd want to switch into the Ghost to use Sucker Punch and/or Pursuit. With access to just Fighting (or Ground) coverage it can cover its bases, however...
STAB that cannot realistically be spammed endlessly for maximum impact – Without coverage for Steels these can be easily walled, moreover because Skarmory and Heatran x4 resist Bug, for example, and many common OU Pokemon weak to one of its STAB would actually resist the other. Another factor is that except for Megahorn (which has low Accuracy) Bug and Ghost don't have a reliable move with Base Power of 100 or more that could be spammed akin to Close Combat.
Lots of weaknesses and resistances – Bug/Ghost takes SE damage from Fire, Flying, Rock (so also Stealth Rock), Dark and Ghost itself. In return it resists Grass, Ground and Poison plus gets Ghost's two immunities, to Fighting (this has overlap with Bug's resistance) and Normal (and this lets it act as a spinblocker)
STAB on notable risky moves – Nothing noteworthy in this field (I don't think Hex counts).
I thought of the Ghost typing ever since the idea of a spinblocker was brought up. I'd have to agree, spinblocking is riskier than spinning, and it invites Pursuiters. I'd like to see this CAP have Ghost typing and see how it fares (notwithstanding the recently made Necturna) - a spinblocker that's weak to Stealth Rock must really make every switch count, but if the risk pays off, the reward is keeping your team's hazards on the field. The tension between hazard damage, weather damage, and probably HP lost for making Substitutes (one has to prepare for the predicted Pursuiter, right?) leaves little room for error, as even a resisted attack that hits it might bring it down once it's at low HP.
I realize Bug/Psychic and Bug/Ghost are very similar in many ways though. A secondary Ghost typing has that spinblocker role on its side, though Psychic offers probably more weaknesses in general (it adds a U-Turn weakness, after all, and only gives a x4 Fighting resist instead of an immunity). As I said in the first line, I'd support Bug/Psychic if it gets chosen, but Bug/Ghost has its perks that just win me over, and I prefer it slightly to Bug/Psychic.
I'm a casual Smogoner that mostly frequents the forums - chances are you won't find me on IRC that often. If you want to talk shoot me a PM please, I'm not on every day but I'll eventually reply.