If we're to be forced into having p&r (urgh), has anyone suggested a system based on average performance over a longer period? I came across just this sort of system in the Argentinian football league - the bottom three teams based on the average points scored in the previous three seasons are relegated - therefore, a team getting promoted finds it much easier to stay up than a team that's been struggling for a longer period.
Probably deemed by some as too complicated, but, as for any of these systems, nobody's asking the fans to do the maths.