Ok, how about an ability that makes it so instead of leaks taking lives and coming back, they don't take lives and go to a random/chosen enemy? That way if somebody is leaking really bad they can give somebody else a bit of their problem. Or if you know somebody will have trouble with a wave and you feel mean, you could sell some towers and use this ability to try to take them out. However normally it would be a gift of bounty for the "victim" as the creeps would normally be very weak by the time they get there. I'm not sure how it should work for elementals that leak while it's active though... Maybe give the "victim" 1 of that element free or 1 vespene if they're maxed, or a large bounty, or just have the ability not affect elementals.