We've gone to a weekday rotation, keeping the weekends in the old rotations.
Admin 1 does Monday morning -> Tuesday morning
Admin 2 does Tuesday morning -> Wednesday morning
Admin 3 does Wednesday morning -> Thursday morning
Admin 4 does Thursday morning -> Friday morning
Rotating weekend admin does Friday morning -> Monday morning
The nice thing is, I'm Admin 5 in the above chart. ;) So I still do weekends but I won't be on call during the week. I will be part of the new Senior rotating admin, to provide assistance to admins #1-4 if they need it. Shouldn't (knock on wood) happen often, though.
We'll see how it works. But it ends up with me doing 4 days less of primary on-call duties per 5 week period, so I'm happy. :)