I've had a thought about PMs for a while now, but I think its just about impossible to make it..
Some way to get/send validation codes on PMs, like.. sending a PM history in a secure way with a number that can be validated on the other side, for proving things people claim to be true.. sounds stupid but would be useful sometimes, but I really doubt its possible, people would come up with cracks and stuff to generate fake numbers
Just my thoughts, not saying it should be made.