Whenever a mission is completed. It would make the map more interesting in some mission heavy places for sure.
I could agree if it happen just for a few minutes like the fireworks.
What happens when several people are doing the same mission at the same time. And won't be able to complete it if one person initiated a beacon.
Why would it not be able to be completed? It could just extend the time? Also if it's a hack any order mission the beacons could be on different portals if they are completed in different orders.
Only one beacon can be active on a portal
I think you misunderstand. I'm not talking about triggering a brain on a portal to complete a mission. I'm talking about rewarding successful completion of a mission with fireworks or some other sort of beacon which will trigger automatically on the last portal of a mission. If more than one person triggers it it can just extend the time it is active.