I have this mechanic:

There are "transdimensional rifts" on some planets. In the late game the "evil aliens from another dimension" will enter and wreak mayhem on the galaxy. The rifts on planets controled by the player will be sealed permanently (after researching certain technology) but all the rifts on other local aliens planets and uninhabited planets will stay open.

Each rift has a certain capacity (how many ships can enter per turn), also the evil aliens have a certain "storage of ships" so these attacks will stop eventually.

The evil aliens will attack absolutely anyone and everyone alive, but they will move into direction of Imperial homeworld (the player's). So, all your traditional local enemies can be uses here as a cannon fodder and a buffer to slow down the real threat.

Now I wonder if I can reuse it for other threats (to save my coding & designing & testing time). Like pirates and rebels and terrorists of some sort (they too "came from somewhere else and you can't reach their homeworld").

Maybe spawn some "pirate bases" on some planets (hidden, need to find out where these are and then strike) and from there they would spawn ships from time to time? Similarly rebels (althrough they should be also able to conquer planets), they could have some "off the scene power rating" which is how many ships they have and then they have bases and from there they can spawn fleets (when they decide to start a rebelion).

Hmmm, I wonder how exactly this should work...

Seems a good mechanic, which can bring some more casualities to the regular game flow. I think it can be a well data driven solution, with some proper behavioral flags, like the "point straight to the player's home planet" one, or "attack closest planet". Maybe a "attack and retire" can describe the action of some pirates forces, while a "continue till total destruction" can describe the behavior of some savage aliens.