I have always had a problem with 4X games, where your empire gets at a point when you know that you simply cannot lose the game and start to ponder whenever it is worth it to spend dozens of hours more just to finish it.
This has happened to me in games like Master Of Magic, Master Of Orion, Rome/Medieval II: Total War and Galactic Civilization II. Somehow Civilization IV had a great balance to not become like this, probably due to limited possible expansion.
In Medieval: Total War slow expansion was achieved by the threat of rebellion within your regions if their approval rating is below certain threshold. Some rebellions could even resurrect defeated kingdoms. Sadly this technique was not repeated in previously mentioned Total War sequels.
In Galactic Civilizations II I only once ran into an event of very serious consequence while perceived to be winning the game. But it is had ridiculous backstory. Pirates A faction named Jagged Knife took over dozens of planets from all civilizations no matter how high your approval rating was with them. But they were a minor race, that was easily defeated by other races. Now we are talking about pirates, not really a splinter faction within a race. But it doesn't make sense that pirates would take over whole planets while acting as a single faction.
I don't have a lot of suggestions though:
- Add the possibility of rebellion within either a planet, race or even between races.
- Add resurrection possibility of defeated races from such rebellions.
These should come out of the blue, there should be some statistic pointing to the possibility of such serious events.