The key to a good game now a days is to balance the butt hole factor. The number one complaint I have heard on multiplayer if people turn it on is the person joining goes crazy killing rivals destroying the ability to spawn the reaper now.
The easy fix is to remove the ability of other players to attack rivals unless the host tracks a rival. Like a switch or something in multiplayer settings.
This should fix it so that folks will open up more multiplayer sessions.