A roleplay set in the Ar Tonelico setting (Ar Ciel). It's a post-apocalyptic setting whose centerpiece is a massive tower/reality-warping supercomputer called Ar Tonelico, with the rest of the planet covered by a sea of plasma the Sea of Death.
So, while the setting isn't original, the interesting thing about the roleplay is the way the world works and how that factors into a roleplay format. Players can play a normal human, or a Reyvateil, which are a race of females with the ability to cast magic via singing (interfacing with Ar Tonelico). Their magic is potentially very strong, but takes time to cast, and Reyvateils are generally vulnerable when singing song magic, making protection necessary in combat.
Magic isn't obtained by your usual level-up, however, but instead, through a process known as 'Diving' where a human dives into a Reyvateil's mind (Cosmosphere) and resolve instances of metaphorical conflicts that reflect their real-life insecurities and mental scars. Diving is a fairly intimate process that requires mutual trust. In a successful dive, the Reyvateil walks away as a girl changed for the better (generally speaking, having your traumas and insecurities cured makes you a better person) and new song magic that usually reflects what happens in their mind/Cosmosphere. The Diver learns more of the Reyvateil; given a male diver, love spouting from this sort of relationship isn't exactly rare, nor frowned upon.
Now, that's enough exposition about the world. You might be thinking, "it's great for a 1x1 roleplay" and you're exactly right. However, the setting can also work in a normal roleplay. Since Diving sessions are inherently 1x1, it would be done through PM, and the GM manages the song magic that might came out from it (preventing instances of game-changing songs from being crafted). When a diving session is concluded, the entire excerpt may be posted in the OOC or edited into an IC post, preventing other players from missing out.
I really want to participate in a roleplay with this setting, but I've never GM'd before.