Just a few thoughts on some of the proposed typings so far
Dragon/grass see jas' post. Seriously though, I don't see this working since you're just adding more grass coverage to a sun team really. The only good I can see from it is an increase in mamoswine which has two underused typings but mamoswine poops on sun besides Volcarona so it would work against us as well.
Water/grass unless this gets some major sun abilities later on, it doesn't abuse the sun at all, which would be fine if its typing didn't lend itself to absolutely loving the rain
Dark I really like this type because it actually goes after a specific threat in lati@s without boosting our weakness to it. I would also love having a good pursuit trapper on a sun team as the most viable at this time is snorlax. Yeah... Snorlax.
Electric I could see this pairing with dark potentially as others have mentioned but I want to look at its merits through its typing alone. First of all electric coverage goes after water types with coverage that has not been present on sun teams since the almighty genesect was banned and he loved pooping on politoed. Secondly, electric gives us a volt turner which, depending upon its secondary typing, would be the only sun volt turner to not have a stealth rock weakness since, you guessed it, genesect.
I would have to put my support currently behind dark/electric because pursuit, volt switch, and their combined coverage is just awesome for a sun team plus there is no SR weakness. However, I would be very intrigued by a typing that would compliment a sun spinner.
EDIT: to the above two posts. A 4x fire weakness is very ideal in my opinion. (just ask genesect) it boosts sun by being weak to it and a good typing could at the same time abuse the sun. However I'm generally opposed to grass at this point seeing as it is very common in sun already, plus SR weakness is nasty. I think a better argument could be made for this than grass/dragon though.