Depends on how deep you want to take it and how many are participating.
We currently do have a Rolyplay General. Mark is a regular there, so she can help you out a bit better with that. But from what I know that story has gone very deep at this point. There are less than 10 users actively posting there but their character roster is already counting more than 100 characters. And although some characters are from the MLP universe (hello again Mark), the story isn’t pony-centered. So if a MLP roleplay is your plan, don’t try that.
In here really depends on the activity, because not everyone here is interested in seeing a roleplay let alone participate in one. If you are only 2-3 people, see if you can hide some stuff below spoiler buttons inbetween posts, don’t make it clog up everything but allow those interested to follow the story. If it becomes more than that, I will have no problem with a seperate thread, but with an already existing Roleplay Thread I cannot guarantee it will last (also making a seperate thread means you’ll open it up to posters who don’t visit this thread).
With how deep the current Roleplay thread is going, it is near-impossible for interested parties to join there, so a second Roleplay thread should not be a bother if it helps things going along. I’m currently a sort of overlooking party regarding those things thanks to someone (hintMarkhint), so I can work it out regarding mods and stuff if needed.