I did a high-fantasy who-done-it in a previous role play forum I was involved in. I had it take place over the geographical area of the city we were in. Allowed for certain types of investigation that you wouldn't necessarily get if they were only able to investigate the building the murder took place in. It allowed for the investigators to look in places that maybe wouldn't be helpful at all. The city had a busy harbor area, so going down there and looking around for witnesses might have been helpful, might have been a waste of time. Perhaps a bribe could have helped them out, or perhaps that bribe was a waste of time and sent them down the wrong path. Because ultimately, one should prove not only who, but how, and why whenever possible.
In my setting I did not have a player be the killer because it was a setting where a player had only one character forever unless they retired it or it was killed off, so unless I had a player actually choose to be the killer it wouldn't have been fair to make one of them play it. Even then, however, there is too much chance of the investigators meta-gaming and making assumptions without actually finding the evidence to support it. As such, I maybe wouldn't recommend having someone play the killer because any thoughts that were presented could be meta-gamed and used to figure out which character it was without the correct effort being put into it.
But anyways, I would be interested in being in such a thread.
I was thinking that all planning involving the culprit would shared between the player and the GM through PMs to keep everyone else from using information they shouldn't have. All info reveals would probably be done by the GM so the player just has to act accordingly to the situation and try to keep the others off their trail for as long as possible. I'll admit that there are probably a few holes in my logic that I haven't noticed yet, though, so feel free to point them out if you see them.
Anyone who wanted to be a culprit would have to make a request by PM before submitting their character as well, and the decision would also be announced the same way, I suppose. Of course, only the player who has the culprit could be told that they are the culprit, and the GM wouldn't be telling how many people sent requests either.
I suppose the GM could just play the culprit, but I wonder if that still makes it too easy for people to figure out. Even if the GM controls multiple characters, it's still pretty narrowed down immediately and I'd like to keep the mystery as interesting as possible.
As for the one character thing...I guess anyone who gets ousted could be allowed another character, but I'm not sure what to do if someone dies in the middle of a round. Would it be alright to just introduce another? We could keep the policy as one character at a time, but depending on the circuit, I wonder how awkward it would be to get them in.