It is a good idea. One that has probably been considered by Bethesda, but how would the mini game work? That is where the problen is.
On the screen, the camera should zoom in to your hand inside someone's pocket. It should start at the top of the pocket, jacket or whatever and not be fully in there.
You should use the left anolog stick to lower your hand further into their pocket and the right anolog stick to move it ever so slightly to the left or the right. You will know that you're about to get caught by the rumbling of the controller. The harder the rumbling, the greater chance that you'll get caught. That's my basic idea. Of course there could be many more factors, but something like this I think would be rather fun and make pickpocketing and thievery much more user-friendly and fun to do.