The term I've seen used most commonly for those kinds of RPs is 'fandom,' though they are spin-offs of a sort so that makes sense.

The issue I see with making some kind of special section for these fandom RPs is how they're essentially the same thing as any other group RP taking place in the Free, Casual, and Advanced sections, just with a differing premise than an original sci-fi adventure or whatever. You see special sections for things like 1x1, Arena, Tabletop, and now Nation RP because they're entirely separate game types that don't really fit in with the normal group thing. The fact that you can describe fandom things as "it's a group RP set in X world with Y goal" and have it understood in the same way as you can say "it's a group RP set in my original fantasy world with the goal of saving the kingdom" shows how similar they are at a basic structural level. For all those roleplay types that have their own section, you can't explain their structure simply as "it's a group RP" or you're missing vital information about the core of how the game works.

As for adding an extra fandom sub-forum tied to each level of group RP, that would just add clutter and confusion. There's also a bit of a slippery slope argument to be made: where do you stop with adding new sorting sub-forums? If fandom gets one, why not high school RPs? Why not have genre areas too? That's obviously solved by Mahz saying "nope, no more sub-forums," but there would still be a lot of demand for each subset of group RPs to get their own special little area if you do it for one of them.

The best solution to the issue of people being unable to find what they want amongst the dross is one that Mahz was already planning on implementing: a tag system with a search function that will allow you to sort by tags. This ought to allow people to tag their Naruto RP as something like "fandom, ninjas, action," then if someone is looking for some sweet ninja action but isn't into the fandom stuff they should be able to search for all RPs tagged 'ninja' and also omit all that are tagged 'fandom.' Given that this sort of system is already planned, I think the whole idea of adding alternate sections to act as sorting aids is something of a band-aid on a broken arm, just awkward and not worth the effort.