So, I through this idea out to my regular server yesterday, after wrenching to death 5 or so spies. If you kill a spy who had sapped your SG, your SG crits for the next five seconds (after becoming unsapped). Requires more metal to build/upgrade. Or, another spy related idea is: Wrench upgrade. If you kill someone with the wrench, your next building builds faster/for less metal (not sure which it should be). However, you repair your structures for less damage with each hit (ie, swing slower).