So we don't wander around in circles for this whole thread, I want to discuss the things I really like and the things I really don't like.
Things I like :)
I believe that CAP 2 needs a strong STAB. I do not want it to have
two strong STABs, however, and would actually prefer if the two STABs did not complement each other particularly well. You may ask me why I would choose to intentionally gimp the typing like that, but I think you all know. Sketch is a powerful weapon, but I really want to encourage CAP 2 to have the possibility of using Sketch for a coverage move. Be this Sacred Fire or Bolt Strike or whatever, I think making players choose between an equally valuable coverage move or a powerful setup move is a good plan.
I also feel that CAP 2 needs to at least be moderately bulky as an extension of its typing so that we can avoid giving it absurd stat-wise bulk down the road. For this reason, I think the secondary type should be something that works decently, but not necessarily the best, defensively. It should open up a few weaknesses, and add a few resistances, but otherwise it should maintain the primary features of the main type without changing too vastly the way it plays.
This is why currently I strongly favor the proposed typings of
Grass / Ghost and
Poison / Dragon. The strong STAB in each is Grass and Dragon. I say that Grass is a strong STAB because OU is not really built with handling versatile Grass-types in mind, and many teams are very vulnerable to Grass-type attacks. Ghost-type complements it well by not hurting Grass's resistances to Water-, Electric-, and Ground-type attacks, while adding useful resistances and immunities and indeed its own weaknesses to Dark- and Ghost-type attacks. Poison / Dragon is similar, with Dragon being a very powerful STAB to have, while Poison really doesn't help it get by Steels any easier. Poison offers a handy immunity to poison and resistance to Fighting-type attacks, but adds its own weakness to Ground as well! See what I mean? If you can create a typing that thinks like these, it may also catch my fancy.
The other real reason I like the attacking types of Grass and Dragon is that they both appreciate the powerful coverage moves that Sketch can provide. This means that when we get to movepool, we can force players to choose between Sketching a powerful coverage move or a powerful boosting move. That choice, given it is balanced as best as it can be, will be a powerful instrument in teaching us about the concept.
Things I don't like :(
Water-type
The Water-type is overly prevalent in OU and very well-prepared for. This isn't the main reason I don't like it, though. The primary issue I have with it is two-fold: The Water-type is a very powerful attacking
and defending type, which is a bit too helpful alongside other types or with Sketch in tow. Secondly, and most importantly, is that the Water-type is directly boosted by rain, which leads CAP 2 to abuse its typing and consequently Sketch far more predictably than it should.
Fire-type
The Fire-type is actually a really good offensive choice for CAP 2, though I find it to be a little too good. I say this because among the best moves to Sketch is Sacred Fire, which with STAB becomes
the move of choice to Sketch on sets. I don't like how this makes Sketch really linear in its use, and so I don't particularly like this type for CAP 2. Oh, and as if that weren't enough, I really don't think we want a sun-boosted type as our primary STAB.
Bug-type
Really, this would be a great choice for secondary typing to a powerful attacking type since it resists Fighting
and Ground, but it suffers from that nasty SR weakness that we have to account for. I hate being "that guy" that doesn't like a type because of a weakness to SR, but CAP 2 needs to be coming in neutral to SR so that it gets room to operate. For this reason, I consider the Poison typing always a better choice over the Bug type for CAP 2.
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As an aside, after reading the arguments for Normal / Ghost presented by Deck and after thinking a bit, I can nearly see the purpose of it. Ghost / Fighting would be the optimal coverage, but you'd have to either use Sketch for your Fighting-type move, or rely on Hidden Power. To that extent, the Normal-type actually functions as the defensive typing I ask for. Still, the major problem is that every Fighting-type in OU outside of the rare Mienshao either carries Dark-type coverage (Conkeldurr, Scrafty, Lucario) or has a secondary STAB that hits Normal / Ghost neutrally (Terrakion, Virizion, Infernape). That means that outside of Gengar (which, admittedly, is really good), there are few real opportunities for a Normal / Ghost to
do anything. This is in contrast to Grass / Ghost, where Grass's resistances give it solid switch-ins against many popular Pokemon (like Rotom-W and Starmie packing weak HPFire for Ferry!), and where Ghost's resistances are in addition to those of Grass. Normal just doesn't offer that to Ghost.
Oh also, I really think the Electric-type might be worth looking into here. I haven't seen any really convincing posts yet, but I think Electric typing as the attacking STAB with something else for the defensive might be really nice. Electric / Poison compounds the Ground-type weakness (Don't pray for Levitate), but is a bit interesting. Some discussion there might be worth having!