There are a few abilities I find interesting... but I strongly think that Drought is by far the best choice we could possibly make, because choosing Drought will help us learn more about Pokemon than any ability for this cap!
I'd like to focus on the bonus questions. These are the kind of questions I've been thinking about if this Pokemon has both Dry Skin and Drought, and are a major reason why I think there is simply no better choice for a 2nd ability on this CAP than drought!
Would a weather Pokemon with a good alternate ability or two use it? How often would they use it?
A lot of arguments against Drought are that it'd totally outclass Dry Skin, but I don't think that's something that can be answered without a test! Sure, Drought is an AMAZING support ability, but Dry Skin is also the superior ability for Sand and Rain teams! Think of the playtest ladder is being a test for sun: Is sun such an amazing weather that this Pokemon will simply become Ninetails but good, or are Rain and Sand so good that this Pokemon is often seen on these teams with Dry Skin? I'd also like to note that if we give this Pokemon a third competitive ability (something like arena trap, magnet pull, or sheer force is what I have in mind), we could further test the merits of Drought here! FINALLY, something big to note here is that there isn't any other weather Pokemon with a good alternate ability, so either way, we'll learn whats more valuable, setting up the weather, or dealing with the weather!
How will Drought and Dry Skin effect the weather wars? Will a Pokemon with Drought that doesn't suck like Ninetails make Drought by far the most common weather, or will the drawbacks of this Pokemon's typing ensure that Rain and Sand are still dominant, forcing CAP 3 to use Dry Skin more often?
Like I mentioned before, in OU, there isn't a weather Pokemon that'll use an alternate ability. And unlike Ubers, the setters up of Rain and Sun are pretty terrible on their own! Sun is also fairly uncommon when compared to Rain and Sand as far as I know, and that's partly because Ninetails sucks, and also because there simply aren't as many good sun abusers as there are rain abusers. With a Pokemon that FINALLY has drought and doesnt suck, we are obviously making Sun more viable. But the question is, how will Sand and Rain teams react? Will we start seeing stuff like Hidden Power Ground Politoed and Psychic Starmie, or will a good Pokemon with Drought not really change the structure of weather teams, or will Sun and Rain usage take a hit because Sun ends up being far better than we expect? Dry Skin is obviously going to be both a blessing and a curse vs Rain teams as well, so if we give this Pokemon both Drought AND Dry Skin, I'm curious to see if Rain teams are forced to radically change their structure in response to two big new threats!
With two such amazing abilities, will Pokemon change their movesets to hit this CAP?
More of an expansion of the last question, but when you give a Pokemon Drought and Dry Skin, KOing it before it causes any trouble is obviously pretty important! I'm really curious how this will change Pokemon's movesets, will we start seeing stuff like Bulldoze Ferrothorn, or will stuff like Dugtrio simply become more common?
Now before I end this post, I want to say that I think Drought fits this concept just fine. Drought obviously makes up for several of the Fire type's flaws, and we can always limit coverage to the point where it still has a reason to use Poison moves! Overall, I think that the potential knowledge about weather wars and metagame reactions to typing make having both Drought and Dry Skin on the same Pokemon far too valuable to pass up!