I see a HUGE problem with some of these types being brought up. Some people are forgetting we've decided we want to reduce not just Water and Dragon, but also Fighting and Steel. While Steel suggestions are not as common, there's too many suggestions going around about Fighting and Dragon types. I just want to post all my thoughts on these along with Grass, so it may be poorly organized but I'm just spitting out everything I can think of.
I'm going to start off by just reiterating what jas originally said. Trying to stop Dragons on the other team using your own Dragon just raises the usage of Dragons. One Dragon doesn't all of the sudden make every Dragon ever hide away in UU. It's like if (for example) Mewtwo got moved down to OU. Everyone would use him, and he'd still get usage in Ubers also, which just boosts the usage of a type. Using Dragon for a counter to Dragons is extremely situational- you'd essentially need to be scarfed with full investment in speed to be guaranteed to pull it off. With the way everyone seems to be posting, this could go either way, Offensive or Defensive, so in the end we could make a Defensive Dragon that was made to counter Dragons. Doesn't sound that logical, now does it? It seems like the only Dragon posting is of Grass/Dragon, which just brings up more problems. We're trying to find a way of stopping Water, Ice, and Electric all at once. Giving a pokemon a 4x weakness to one of them isn't helping. More on Grass types later.
I have some extreme reservations about this type. You could argue for days about how Fighting is the best type behind Dragon, if not the best type. While it may have some potential towards rain teams because of an SR resistance, I still generally don't like the idea of using a Fighting type to stop common types, seeing as it is one of them. It's like a lesser version of the Dragon situation, but most fighting types hit so hard with STAB moves that other Fighting types not named Gallade, Toxicroak, or Heracross can't reliably come in on without being threatened to get 1-2HKO'd. And honestly, the only thing you're getting out of a fighting type besides an SR weakness is the ability to stop Ferrothorn and Hydreigon only if it is for some reason chosen over Lati@s. Other than that, you're destroyed by STAB Psychic moves Lati@s might have, most likely crippled by a Scald to the face from Politoed or whatever Bulky Water they're probably running, and really only have something going for you if you're grass typed also, which again will be talked about later. I could MAYBE deal with dragons only because they usually get great coverage and resist Water and Electric, but Fighting doesn't do that. Fighting does have the Rock resistance for Stone Edge from Sandstorm teams, but it doesn't resist Ground anyways and once again is really relying on it's dual typing to give it some backbone.
While I've only seen a few suggestions of these, it kind of concerns me that we're using a water type on a sun team to counter rain teams. Honestly, if it's good enough, rain teams are also going to use it anyways just because it's Water typed. The only suggestions I saw for water we're Water/Grass and Water/Fire, both of which could operate on Rain teams just as well as sun teams. I don't have much more to say on Water, pretty sure it's not getting anywhere anyways.
Steel is probably the "Big 4" type I mind the least. It's not being brought up much anyways, but I just want to say it's neutral to Water and Electric attacks and resists Ice. Somebody could probably come up with something good here.
I have the most concerns when it comes to this type. Adding another Grass type to a Sun team is not going to help. It's just going to compete with Venusaur for a team spot, and if Venusaur beats it out the project is lost and a complete waste of time. The only way anything Grass typed is going to get a team spot along side Venusaur is if it has so much pumped into its Defensive stats and has Leaf Guard or some defensive ability, or is so amazing at utility to the point it can set every hazard, spin them, have heal bell, and roar/whirlwind at its use. We know all these most likely won't happen. Grass typing is taking a HUGE risk with the project. It could become a worse Venusaur. Everyone might use it anyways just because it's a CAP Pokemon, and it could just open up some other problems in slot syndrome where you're losing valuable coverage or resists just because you wanted to throw another Grass type on the team. I would prefer if we stay away from Grass completely and fully.
As of right now, I'm giving all my support to Dark/Electric. It's got the ability to threaten Water types, trap Lati@s, hit Scizor neutrally, still somewhat threaten Sandstorm teams outside of Ground types, and is not weak to SR.
EDIT: Like what Deglas said about Dark/Electric too.
Contact me if you need monotype help, I'll be more than happy to oblige.