Role-players amok in Second Life
Games within a game
Making it up as we go
Other RP opportunities are more free-form, such as street combat, medieval fantasy, capture, and various forms of sexual exploitation, where there is no corpus of literature or TV shows to draw upon. Often, there's little more than a general agreement to the type of scenario that will be played. Midian City does have a culture and a background - a back-story, if you will - but it still allows for considerable freedom and spontaneity in play. Oh, and if you like Midian, check out Toxia, aka Toxian City, and City of Lost Angels. The three are somewhat similar, but the differences are enough that if one isn't quite your cup of tea, another might well be.
Some RP sims are really just physical environments for various forms of play. One, called CARP, is based on capture and BDSM, and is nicely set up with mazes. Players choose to be predator/Dom or prey/sub, and the prey characters run through the maze attempting to elude the predators. The maze works for, and against, participants of both sorts. For example, a target will find it easy to disappear momentarily and make a move where the pursuer will need to guess the right direction; however, because the maze is elaborate, prey characters are often turning corners and running into other predators.
When predator and prey collide, the prey is automatically captured and cannot move unless the predator chooses to release them. Since the capture script is triggered by a collision, accidental captures are common when the maze is crowded. Normally, a person caught accidentally is released. But in the event of such a capture, the two parties can certainly have a conversation and decide whether the accident is a happy one for them.
CARP is fast paced and not very talky, at least until after the capture. In the maze, it's more like a video game than RP, but there are rooms with various scripts and equipment for post-capture role-play involving bondage, torture, rape, etc. There's also a lounge where visitors can chat and learn more about it - and the place is exceptionally furry-friendly, which I always take as a positive sign. (Actually, I think it's under furry management, although it is used by quite a few humans.)
For something in a similar vein to CARP - although a good deal less anonymous - do check out Drifter's Passage. This is a much smaller, and far better-looking, kidnap and capture community where it really pays to take your time, get to know people, and develop a character with connections to others in the sim.
A lot of Second Life RP involves BDSM, or is at least influenced by it to some degree. It is one of the most pervasive themes running throughout SL sex - and since sex occupies people's attention a great deal, BDSM, or hints of it, turn up pretty much everywhere on the "mature" grid. A sim featuring schoolgirls trapped in a decaying reformatory with cells and torture rooms (there are two rather large ones, btw) might not sound like proper BDSM, and it's not, really: it's better to call it a BDSM-influenced fetish RP. Strictly speaking, BDSM is a lifestyle, not a fetish or an RP game. But there are fetishists and role-players who draw a lot from the lifestyle - and they do seem to be everywhere in SL.
Of course, there's a lot more to Second Life RP than torture and rape. If you search Places and Groups for "roleplay" you'll find scores of places to explore. But don't make the mistake of assuming that a role-play sim is necessarily good because it's ranked high in the search results. Search rank is connected to traffic, but "traffic" is hardly the same as involved, regular participants. A lot of the "traffic" in a busy sim might visit occasionally, play a bit, and return only when they can't find better things to do.
So don't be reluctant to look further down the search list. Quality, popularity, and search rank are not always so well connected. Overall, you'll find that "street" RP is very big, Medieval, faerie and dragon are as well, vampire and neko (cat) are common, and of course, there's every manner of sexual RP, particularly involving abuse - and including age play, which I'm afraid I simply am not inclined to investigate. You'll just have to rely on your own imagination for that. ®