A good, solid, historically-based, nightmare-inducing Jack the Ripper adventure represents to me the Unholy Grail of CoC gaming. I feel like if I could pull it off with aplomb, I could probably retire my CoC stuff permanently. Here are the features I want the game to have:
1) A sense of period authenticity.
2) Use of historical personages for as many characters as possible.
3) Attempt to adhere to the Ripper timeline.
4) The PCs are all people nowadays suspected of being the Ripper, even if they weren't back then. The idea here being that these folks are suspects because they were unofficially investigating the crime.