Relevance would be calculated as the ratio of one's reputation over the number of one's questions and answers, times the number of views —EDIT— per unit of time. (I would not include comments because it would refrain people from commenting.)
It would be equivalent to finding out who are the most efficient users? Who generates most reputation or impact or bang out of the fewest amount of questions and answers (and in a sense time)?
User relevance leagues would promote more pertinent questions and answers. It would also set an achievable goal to people who cannot or want not spend all day on a specific SE site. Moreover, it would serve as a motivator to both generalists (those who spend also time on other SE sites) and newcomers alike.