First of all, I would like to say that I oppose unreleased Drought. There is little to no merit in giving Mollux an ability it can never use. Frankly, we may as well give it no third ability. It amounts to the same thing, minus the chance of getting annoyed by people constantly asking "waaaah, why can't I use Drought?" And we have no way of knowing exactly what Gamefreak's plans are for upcoming games and events; how would we explain Mollux being the only mon in the entire CAP metagame without its DW ability released, should it come to that? Our alternative would be even worse: releasing Drought, against the wishes of the community who voted it down not once, but twice.
I would prefer to see a third (released) ability that has at least a slight bit of competitive merit, albeit one outclassed by Dry Skin. Granted, this means Illuminate would get no use whatsoever, but I don't see that as a bad thing. I would like to make the case for a woefully under-explored ability that I haven't seen suggested to this point: Wonder Skin. The only other fully evolved mons with this ability either have an ability that is vastly better (Tinted Lens, Magic Guard) or are Delcatty. I believe this complements Dry Skin very well flavor-wise, and makes sense for Mollux in general from a flavor perspective. While Dry Skin is certainly better overall than Wonder Skin, it could see use on sun teams. The lack of 100% reliability helps keep it in check, while at the same time increasing Mollux' resistance to the few statuses it isn't already immune to.