Dogpile Initiative

Another variant of this is the current actor chooses who acts next, this includes the enemies. Dog piling can happen at first but your party better be damn sure they finish in the first round otherwise the enemies will all choose each other and get 2 back to back rounds. If you put them up against dangerous enough opponents then they will naturally want to ensure they can react between opponent turns.

Then as long as your players are playing nice and giving your villains turns then you have the opponents play nice and choose players to follow them.