I'm going to bed very soon, but I wanted to make a quick post here. We've had an incredibly engaging conversation in #cap for about the past two hours, and I wanted to post a bit of what we gleaned there in this thread.
We talked a lot about waters versus dragons, and didn't make a lot of headway. Yes, both types threaten sun. In general, many sun teams gear themselves in either one direction: they either handle dragons or go for trapping toed and ensuring sun. The ultimate problem of sun is that it lacks team slots. In order to function well, you need to run Ninetales / Venusaur / Spinner / Trapper / Slot / Slot, where the last two show some semblance of a defensive pivot. So what would be the most beneficial thing we could give sun? Giving it more team slots would ultimately give it the flexibility to incorporate more Pokemon. If CAP 5 was able to trap (this doesn't need to be an ability!), keep Stealth Rock away, and serve as a reasonable defensive pivot, then sun has around three slots open to utilize to bring down Water-types and Dragon-types.
Many sun teams have to utilize Venusaur as a check to bring down rain teams; unfortunately for them, Venusaur is also their premiere sweeper. If it dies, then the team loses a key element to its offense. If we can harness that same idea of checking rain teams in a pinch, that saves sun teams yet another team slot. I'm not saying replace Venusaur, but distribute some of its role to CAP 5 to give Sun more options.
Sure, we can scurry around here and try to counter a few specific Pokemon with CAP 5. The thing is, those Pokemon already exist. We already have counters for the Latis and Politoed. If we want to threaten dragons and water, we need to make sun better (I thought that was the entire point of choosing sun mon). So working to consolidate some of those necessary roles on a sun team is the biggest buff we could give to a sun team. jas61292 has made clear that we shouldn't try to do everything with one single Pokemon. I agree with that, and I don't think I'm asking for that much here. All I'm saying is that we should have a) a way to come in on a few common threats to sun, b) a way to deter/remove stealth rock, and c) a way to trap other weather starters. In doing so, we've effectively given sun teams four slots to do what they please with.
Now, in order to do point a) that I listed above, we should focus on a few Pokemon here that sun teams would like to have some solid switch ins. The reason I support Water-resistant types is because we can't really have a Dragon-resistant type without it being Steel (ugh). I don't care if dragons or waters are more threatening to sun. What sun needs it a Pokemon that can start filling in some of the roles of its team. By proving a water resist, we free Venusaur up to perform its role as a Chlorophyll sweeper, and therefore more team slots are availabe. So yes, let's try to push for something here that frees up some space.
The type I'm going to push for now is Electric / Grass. It has that solid water resist that Sun teams can rely upon, but also some other useful tricks. Notably, it provides Electric STAB for Sun teams, which is basically unheard of outside of Zapdos and Thundurus-T (which I don't think many would consider sun staples). In fact, it hosts STABs that force out all three big weather starters. I also like Electric / Grass because it's not blatant about trying to do it all. It doesn't fare well against dragons or all rain threats, which leaves us for some other things to tackle down the road. Finally, it doesn't really offer much on rain teams on its own. Plenty of other Pokemon outclass it, notably Ferrothorn which can set hazards and has an incredible typing. We can certainly gear CAP 5 with this typing to be outclassed on rain teams, yet preferred on sun teams.