This is something I noticed during my genocide run. If some truly unstoppable force of nature started tearing through the galaxy (either nuking planets or crazy market shares like 50%), then I think despite each faction hating each other they would put their differences aside temporarily for a common goal to destroy the monopoly faction.
The notification could look like "Hegemony-Luddic Church coalition formed" and so forth, and then there'd be some flavour text about the reason why, similar to the flavour text when factions raid your system.
It would be the same mechanic as the hostilities system but in reverse, so perhaps when the coalition is formed it would put the reputation for both factions at 0 and then any infractions - even minor ones like fleets not having their transponder on enough times - would reduce it until the coalition is broken around -25 rep or so. I think this would be pretty easy to implement and make the game world much more dynamic.
