Currently the only RP is DCW IRL. Yes? That is a start and end affair. The only purpose it would serve is to act as an archive for past plays. But you know what? Site search does the same. Since searching for "DCW IRL" will serve as essentially the same thing, if you really need to look up old threads of the RP.
As for everything else, that doesn't actually change my point, whether it is DC related or not. There need to be general rules, because it would branch out (in other words universal RPing rules that won't be breached, which would maintain fairness). There would need to be special moderation. There would be constant moving. The needs of RP forums are totally different from most other forums.
There is a reason why most forums don't have RP even though the fandoms clamor for them (and those that really want them, tend to create their own forums specifically for RPing purposes). Because of the work and the drama they bring. Most forums keep it to communal discussion, and all others specify that they are an RPing forum.
DCW IRL is fairly contained as far as I can tell, so asking, for perhaps a log area for past RPs I understand more. But a whole other RP subforum? Not so much. However, it would be specific to DCW IRL. The second it expands past that problems occur again.
There are things to consider if they are long spanning or spread across many threads. First is how to keep track of everyone's character. Character creation is one of the core essences of RPing. Character logs: how to keep track of what each character has done. Further subforums if it does take off. Unless you want the new RP subforum to become clogged and confusing with multiple RPs going on at once. Constant moving of threads, taking out old ones and moving them to inactive (unless of course you have sufficient subforums created).
Clearly you are only thinking short term, which is ease of use of DCW IRL. Which can easily be remedied by methods previously stated. Long term RP forums take a lot of upkeep. I'm not sure you realize that. If it grows, like you hope it will, it will only become exponentially more trouble as time goes on and it gets more popular. As it stands the RPing community here isn't big enough to warrant a separate subforum. This why Carpet Crawler brought up DCTP (or so I assume). DCTP has just as large, if not a larger RPing community than DCW, yet does fine without an RP subforum. Why? Because it is simply not needed for the amount of RPing that actually occurs, and would only bring more complications. The RPs are self-contained and searchable if you want to go back and read.
Leave it at that. This is simply not needed. In essence, you are asking for the admin and mods to take on additional responsibility and work, just so you can find a few threads a bit easier, Use the search function. It is rather simple.
tl;dr: It isn't like there are many different RPs here. There is only one. Use the search to find old threads, and problem is solved without the creation of a new subforum, which can potentially bring more problems.