I like Mark Steyn as a substitute host but he doesn't wear well on me. Rush is the only commentator that I can listen to everyday without tiring of him.
A random thought. Has David Limbaugh ever substituted for his brother? I really like David Limbaugh's writings but I don't recall ever hearing him speak. Probably not a viable option to take over for his brother if, heaven forbid, that is needed in the short term.
Mark would do best as a host two or three days per week with guest hosts and interviews.
He is a great interviewer and can showcase the best of conservatism as well as the ragged edges.