I wanted to run a short mystery adventure. Mysteries can be tricky to run, particular when the focus of the rule set is combat and spells. I ran The Melford Murder several years ago, a Dragonsfoot adventure module written by Stuart Marshall that I used in a Castles & Crusades campaign. Not we have blast playing it, I also gained some valuable experience in running a mystery adventure. Playing The Esoterrorists, a GUMSHOE-based game, was also a huge help.
It is actually easier to run a mystery adventure with a simple non-mystery rule set, like Pits & Perils, than it is with a more complex game, like D&D 5e. The characters have few abilities to provide an unwanted shortcut, so the players are forced to strategize and discover solutions.
I will post more once we have finished with this part of our round robin. I can't really say much about how I am doing it without tipping my hand about the mystery itself.