Bearing in mind that the concept behind CAP5 is to "rebalance the type chart", I think it goes without saying that Pokemon with conventionally unpopular typings should be able to do well against it, and Pokemon with popular typings should do poorly against it. Failing this, CAP5 should at least be able to create good opportunities for its teammates to deal with Water, Dragon, and Steel types. For now, I'm just going to suggest some Pokemon that we could make excellent checks/counters to CAP5, and thus promote their typing's usage.
Infernape: Infernape has a pretty bad rap in the BW2 metagame, mostly due to Rain being everywhere and Keldeo being a much better sweeper/cleaner in the most common conditions. However, two things should be noted; it resists both of CAP5's STAB attacks, and becomes significantly better in a Sun-friendly metagame. Making Infernape a better choice on teams, not only to deal with CAP5 but also to act as an offensive partner on Sun teams, would be a good way of promoting Fire-types.
Volcarona: Another Fire type, would take CAP5's Grass attacks like a champ and could cause a Burn with Flame Body if CAP5 opts to use physical Dark attacks. Boosting Volcarona's usage would not only encourage the use of more Fire-types, it would also provide Rain teams with a reasonable defense against CAP5, as Volcarona's great in both Sun and Rain, and can OHKO with Bug Buzz either way. Allowing Volcarona to do well against CAP5 would be important in ensuring that CAP5 doesn't completely tip the balance over to Sun. That said, it'd also be a great teammate choice, especially if CAP5 gets Rapid Spin.
Toxicroak: While Toxicroak only ever sees use on Rain teams, again, making Toxicroak a good response to CAP5 would ensure that Rain has at least some chance of staying level with Sunlight, while also providing some good type diversification, making it less tempting for Rain teams to simply fill their ranks with Water and Steel types. Like Infernape, it resists both STAB attacks inherently, so it could probably do a good job. Poison types are also pretty rare, so boosting them would be pretty cool.
Crobat: On a similar note, here's another Poison type that could do really well against CAP5. Crobat has a lot of ways in which it can deal with CAP5. 4x resisting Grass moves, almost certainly outspeeding it unless CAP5 has a Scarf or some kind of Speed boost (Chlorophyll?), STAB Brave Bird or U-Turning for a lot of damage. Crobat isn't really helped or hindered by weather, but getting a bonus on Heat Wave could give it at least a shot against Steel types.
I think that ensuring that there are suitable non-Steel-type ways of checking and countering CAP5 is important for fulfilling the original concept, so finding ways to make Poison, Bug, and Fire types such as the above more suitable responses sounds like a good idea to me. Other cool threats could be Houndoom, Heracross, and even stuff like Scolipede. Personally, in terms of dealing with Steel types, I think a Sun-boosted Hidden Power Fire or possibly Fire Fang would be good enough, as it lets CAP5 deal with stuff like Scizor, Forretress, and Ferrothorn, while leaving bulkier, non-4x weak Steels to Ninetales or other teammates. This would also allow the aforementioned Pokemon to more reliably check CAP5 than if we give it Ground or Fighting coverage, although it could still use other Hidden Powers if desperate, eg vs Heatran.
Check out Slowmons: A new metagame where the slowest Pokémon moves first!