A lot of people like Grass because it resists Water. However, this thinking focuses a lot more on defensive Waters than offensive Waters. I feel that the latter is more threatening to sun (as it currently is) than the former, because not much in sun cares about Scald (except physical Venusaur, Dugtrio and I guess I'll include Sawsbuck in there too), while offensive Waters will use coverage moves to tear a Grass-type down. This, I think, is a serious thing to consider when talking about a Grass-type CAP 5. I'm not saying Grass would be bad, since there's certainly something to be said about relieving Venusaur of multitasking pressure, but it is something to consider.
To add onto what CiteAndPrune and jas61292 said, I don't think that anyone's goal in supporting Dark / * is to shut Lati@s down. I don't think that that's all that realistic. However, with stuff like SubCM Latias being what it is against the typical sun team, I definitely think that at least deterring Lati@s would be beneficial to sun teams. It doesn't try to do too much, which would risk making an overall great Pokemon that doesn't really help sun. I think that the late Genesect is a pretty good example (not perfect for obvious reasons) of what we should be aiming for here. Genesect did generically help rain teams, but it also helped sun teams, making teams like Lavos Spawn's GeneSun a significant force in the metagame, without being "broken" or whatever.
I don't think that we should address the questions of "biggest weaknesses" or "role-filling" so generically. We can easily slap Ice Beam and/or Ice Shard on any typing and call it a day (or at least, come as close to doing that as might be reasonable) against most Dragon-types and Ground-types. We can easily slap things like Rapid Spin or Will-O-Wisp or a host of other things onto any typing as well. To me, it's more about threats that we would like to address but might be harder to address than other threats without heavily involving the typing.