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.