Here's something that came to me while I was doing missions on asteriods.
Sector Space:
How about random appearing comets/asteroids in sector space? I mean those are physical bodies passing through space at all times.
Make them a fleet action, or make them a raidisode or make it even a single player mission I don't care. But make them challenging and with good (rare loot on very rare occassions also "very rare" loot) loot and a bit longer than the standard story missions. Something from 20 min. to max. 1 hour?
They'd be in sector space for a set time and keep moving from one direction to another and after that just disappear.
I mean that wouldn't be too hard to implement, no? It's the random exploration system coupled with the enemy sensor contact somehow.
Just an idea I thought I'd trow into the crowd.
PS: Doesn't have to be asteroids/comets specifically. Can be unknown contact, unknown entity, strange reading, whatever you call it.