In short, a genderbending smut fest. For sure I will not be genderbending Celestia, but this could possibly include a lot of different characters, as well as making characters just for the purpose of furthering this.

The alicorns of the realm are usually the symbols of nobility or even divinity, but it isn't an odd idea that they get up to nasty business behind their royal lives.

(I wish I had a better description but I am having just a little bit of trouble.)