Yeah, my first idea was to make a Facebook game, so:
1) you should solve it as fast as possible making less mistakes as possible in order to brag to your friend
2) everyone has to be able to solve a case in a reasonable time.
So, my idea is that, when a case is designed it should contains all the evidences of the murder, the weapon and the motive.
so, when a player makes the wrong choice the system tells him an evidence which should fix a part of the solution.
I'll give an example... let's think the user is playing in the scene from "A study in scarlet" of Sherlock Holmes,
if he makes the wrong combinations about murder,motive and weapon the system says:
"There is no blood of the body in the scene, so the weapon is poison"
if the user makes a second wrond combination the system'll says:
"There is a wedding-female ring in the floor, the mandatory is the gealosy (there aren't sufficent proofs about the woman's name)"
if the user makes a third mistakes:
"the telegram from Cleveland says that the dead man had requested protection from a rival in love that has followed him from America to Europe, his name is called Jeferson Hope, and he is the murder."
So you have 3 chance to solve the case since the 4th time the system has told you all the necessary information to solve the case (showing all the evidence).
I think the score have to be related to the solving time but mainly to the mistakes made.