Ok, so here's an idea I came up with recently.
What if separate team leaders could request peace or an alliance with another team, so players on both sides cannot attack each other.
This could be useful in a variety of situations, particularly as the leader would have to end the peace agreement, allowing players on both sides to know that the alliance was ended.
This could be a command that the team leader uses to send a alliance request to another team, and that teams leader accepts it in order to prevent PvP between the two teams.
TL;DR A command to ally two teams to prevent PvP between them