I suppose the closest model we have for W is Nixon...who pretty much went into seclusion after he resigned in disgrace. Nixon, at least, became a bit respected for his foreign affairs perspectives and wrote several books. W will be best remembered for his lack of foreign affairs perspective and perhaps has never read a book.
I think its fairly certain that he will return to Texas (not Crawford, which Laura hates, but to Dallas). Perhaps he will lecture to SMU students, make a few appearances at conservative and religious conferences, and show up for funerals...but I just don't see much appetite or demand for him in the public sphere.
Many presidents like Nixon and Hoover have been judged by the negative actions they had on the country during their administrations. W will be judged not only on how he fucked the country during his term...but he will also be remembered for a generation on how he fucked over future generations through debt, energy policies, the bank failures, and other ineptitudes. He is nearly unique on that fact. Way to go W.