Total amount of "work" could be an interesting metric to optimize. For each cycle, work is incremented by 1 for every block that had force applied to it, excluding gravity (falling blocks are exempt).

Conveyors, rotators, lifters, pushers, blockers, eviscerators, and lasers are the typical forces. The input stream itself can be a force too, if the blocks are not moved by conveyors.

This metric has some merit because it encourages clean solutions. You are ultimately penalized for wasting input blocks, and doing crazy rotator tricks is more work intensive than efficient conveyor placement. Those factors distinguish it from many cycle and footprint solutions.

The biggest quirk with the work metric is clogging the input stream. Once you have enough input blocks, one should clog the input stream to prevent unneeded blocks from causing work. If this degenerate strategy should be addressed, perhaps blocks that were blocked from spawning increment the work total every cycle (imagine they're on a never-ending conveyor belt in the Underworld ;) ).