The game becomes too easy after you have a good colony or two, with unlimited income and a strong fleet there is no danger anymore, winning becomes a simple factor of time, and at least for me, it becomes too boring, i always restart after taking down a faction or two.
So, what if there was a player threat level, every great deed like successfully invading a large world, betraying a faction or expanding colonies too fast would raise your threat level, this would amplify all rep loss with other factions and make them send bigger fleets after your colonies instead of sending sad little scraps that get obliterated in orbit every time, also enemies joining in alliances against you would become more common, after all a nobody who in less than 3 years forms a faction and destroys the hegemony would be a danger to the whole system.
Obviously there are people who enjoy taking their time and playing slowly. This could be an option you set when starting the game.
