jas61292 and I have had quite a conversation about this, but I want to make an opinion clear on this. The crux of the discussion is "what is the primary threat of sun teams" and that guided our conversation. His opinion is that dragons are the most difficult hurdle for sun teams to overcome, specifically the Latis. While I do think that sun teams have problems with dragons, they typically have strategies to overcome them. There's Heatran (and Forretress, Chansey, Gliscor to a lesser extent) to help sponge attacks. They also have faster Chlorophyll mons, which can utilize coverage attacks like Rock Slide and Sleep Powder to bust through. Finally, sun teams often run a scarfer, so there's at least a way to respond for the revenge kill. While I do agree that dragons are a threat for sun teams, I can't help but think that sun teams have already developed quite a few strategies for handling them.
In my opinion, rain teams give sun the biggest headaches. Arguably the biggest thread is Politoed itself. What on sun teams reliably switches in to Hydro Pump? They are either forced to use a sponge such as Blissey or a specialized mon such as Gastrodon. Outside of that, Ninetales is probably your best bet. And between SR and Hydro Pump damage, you're not sitting pretty. In fact, Ninetales just doesn't do well against any member on a rain team sans Ferrothorn and Toxicroak (kind of). And when you add in Keldeo and other rain threats to the mix, sun has a serious uphill battle. The onus is basically on the sun team to trap toed and ensure sun, or they can expect Ninetales to be worn down and eventually taken out.
So what motivates me is this: we can handle dragons during other CAP stages, but we cannot sponge water attacks at any other stage. Theoretically, we could stop rain-boosted water attacks by adding monstrous special bulk or a Water-type absorbing ability. But outside of those two options, we can't really resolve the constant struggle that sun teams have with rain. On the flip side, we can stop dragons at many other stages. We can make CAP 5 faster than them, or give it the correct moves to take them out. I feel that if we wish to oppose rain teams in any sort of format, we should consider it now; stopping dragons can come later.
Therefore, I am in full support of all Water-resisting types. That narrows us down to Grass, Dragon, and Water. I think all three of these types would add something interesting to sun teams, especially when combined with another unique typing. None of these types are SR weak either, which is an added bonus to sun. I know Water-type seems really counter-intuitive to help a Sun team, but sun teams don't really have a go-to Water-type. Regardless, it's something to consider. The issue jas61292 has brought up with these three types is that they don't bring anything "new" to sun teams. To rebuttal, I don't think you really need a new typing to add something new. Volcarona doesn't add anything new to sun teams with its typing (except an even bigger SR weakness), and yet it's a staple due to its great movepool and good stats. I don't think we should limit ourselves from utilizing these three types because they're not interesting or unique.