I'm not impressed by Willis thus far on or off the field but I do believe he has the physical talent to be very good. The heart and the mind I am not too sure about. However, young immature players like Willis often grow up during their NFL careers so maybe the lightbulb can still be illuminated for Willis. If I'm Marv, I'd approach a contract extension as follows.
Write a new deal that extends him a few years. Instead of front loading the money, put in a big roster bonus each year. That way he's playing for a contract each year. If the guy gets his head on straight and consistently performs, he stays a Bill and gets paid very well. If not he can be cut or traded without salary cap baggage and he becomes an UFA. The player and agent won't like it, but it's fair to both sides.
I remember when Jerome Bettis was run out of St. Louis. His behavior and effort early in his career was poor. Over time he really became a class act on and off the field. Had a pretty good career in Pittsburgh. Would hate to see Willis run out of town only to right the ship shortly thereafter. I'm not sure he has it in him, but if he's motivated the right way and get's some mentoring he might surprise all of us. If I were him I'd start by getting a new agent. One that coaches his clients better on the "taking the high road".