Hats off to Scorpio for making me not feel entirely alone in my thinking. <3
Anyway, I'd like to point out a few things that I'm not sure everyone realizes, save for those frequenting #cap.
Our typing needs to not only be good in opposition to rain, but should also be incompatible within other weather as well. This being the case, we don't want our typings to be Water or Rock (not that anyone is even suggesting the latter) or else we risk making a CAP that could thrive in Rain or Sandstorm and thus wipe out Sun's usage. The exception to this is Ice I suppose, as Hail is such a minority weather as it is, and bringing up the amount of hail teams would in term make Fire-types more valuable for countering them.
Stealth Rock is one of the major issues with sun-teams, meaning that if possible, we should select typings that aren't Stealth Rock weak, damage neutrality at the very least. This being the case, Fire and Ice-type suggestions are fine, but the switch-in damage it will take is going to compound an issue that sunlight already has. Consider that when arguing for your type.
While Lati@s is a major boon to sun teams, basing your typing to being part Dark or Ghost simply to counter them is a little extreme. I've heard your arguments for them and they're solid points, but I still feel like its use is a little too niche to really benefit us. More relevant offensive or defensive types should be considered so that we can handle entire weather playstyles, not just two Pokemon. Additionally, there are plenty of Fighting-types that would smack down a given Dark-type CAP5, and Tyranitar would simply tear apart a Ghost-type. Countering the Lati's is fine, but we can't pigeonhole ourselves into being a dedicated check to them without risking losing the weather wars in general. If we focus on killing Dragons and Water in general, as well as keep in mind compatible teammates that can pick up on what CAP5 can't handle itself, CAP5 will do a much better job at fulfilling its concept.
Also, perhaps it's just my own taste, but no Dragons, please. We already have plenty of those and we're trying to lower their usage. Including Dragon-typing might actually increase the usage of Dragons on part that we're weak to them. That would be counter-intuitive to the concept.
So far, the most solid typings to come out of this thread sound to me like Grass/Electric, Electric/Fighting, and Ice/Fighting, if I couldn't make the choice of my own proposed Electric/Ground. Of course, that's just my opinion. You have no obligation to care, but I would assume you would have some degree of agreement with my reasoning if you've honestly read all this.