I'm pretty certain at this point that the next novel I write is going to be a mystery of some type. It might be an alt-history cyberpunk time-travel conspiracy-theory mystery (or some such), but it will be a mystery. I've already been inflicting whodunits on my roleplaying group; I might as well try to write one for real.
There are two things I like about mystery stories, the first being the problem-solving aspect. I like solving puzzles, and even though I'm terrible at solving a mystery story before the protagonists do, I get a vicarious thrill out of watching them do it. I like to watch the heroes discussing possibilities and spinning out theories to connect the known events.
Second, I think that mysteries present an interesting way to perform character exploration in fiction. In a good detective story (or at least what I consider to be a good detective story), when the sleuth interviews a suspect, the interview provides more than just information. The interviews reveal the personalities and the motivations of the suspects, as well as their perceptions of the other characters, by way of dialogue. The author can use these scenes as opportunities to deliver a concentrated dose of character development.
As such, I expect my hypothetical mystery novel will be more of a crime story than a whodunit, since I don't expect to lay out an intricate web of clues that would lead the observant reader to an inevitable deduction of the identity of the guilty party. I expect to set my protagonist on a search for the truth in the middle of a pack of characters who are all lying about something, for their own reasons, and let them all flirt and fight and scheme until the gunsmoke settles and the facts are revealed.
Maybe instead, it should be a historical coming-of-age urban-fantasy romantic mystery...