My club seems to work pretty well. We split into groups based on speed. As rides below A are "no drop" and have sweeps. We have short, medium and long courses. On Tuesday nights we have
AA: 20+ mph
A: 18-20mph
BB: 16-18 mph
B: 14-16 mph
C: 12-14 mph
the reason we don't have C and D rides is that they ride on Thursday nights. Usually out "B" group is the largest (20+ riders), so usually gets split into a B and B+ group.
This system works well and seems to make riders of all levels happy.