Give JW a one year contract and review it on an annual basis. If he doesn't produce, cut him loose.
What assistant would take that deal? Would you take a 1 year deal with a company and review every year if you have a family? As a coach you want stability, if Illinois offered him a 1 year deal to "prove himself" even after he has proved himself for a few years here, I would fully expect him to leave. I would hope he would leave actually, he deserves better thant hat.