Team Finishers

It's could certainly help with the problem of finisher steals. Maybe if you press the button at least half a second after another execution begins, you perform a co op execution.

Especially on the boss at the end of a mission, having all three players perform the finisher could be awesome. But just to keep a third player from griefing and just extending the mission unduly, there could be a limited window of time for the third player to join in, before just performing the execution with just the ones who're participating in it.