I thought we were talking about the evolution of the game to include the 5th X. I am specifically targetting Space Empires V ideas
The idea is to provide some extra spice to the game at the point where it seems to lag. In the case of SEIV, it lags when you reach MEE. So, MEE status is an event that can be used to trigger something that adds spice to a game that has begun to lag. In addition, the event needs variety or it can be defended against. For instance, in the Civil War Event, the player would know that inside the next xxx turns, half of his empire would secede, he could adjust production, storage, etc so that he would be able to choose an optimum set of planets to retain. If the player knew that Barbarian Hordes were imminent, he could stockpile partially built ships and release them when the Hordes landed. However, if he knew one of those were going to hit, but not when or which...then, it'd be much more difficult prepare for.
The idea is that its a random calamity that is potentially disasterous and that is targetted at the MEE Empire. This could actually save the PLAYER if one of the AI's got way ahead of him.
The Barbarian Hordes is a particularly good idea because: 1) It will eventually run out of ships and end the disaster...usually before wiping anyone out; 2) While it starts in the middle of the MEE Empire, it doesn't necessarily have to stay there; 3) It is not unsurpassable technologically and therefore can be beaten back.
The Civil War idea could be a bad idea unless you gave him a chance to prevent it from happening. Perhaps you could send a message to him saying "You have reached MEE status. Your Empire is in unrest. You must quell the unrest or suffer a Civil War." Quelling the unrest would require ships in every system....say a number equal to the number of colonized planets & mooons. You would have a time period to achieve this and if you failed, then you would lose half of the systems (you'd get to choose which ones you get to keep) that did not have the appropriate number of ships in orbit. If you DID achieve the goal, then you'd have to maintain this navy in each system you occupied, or go into a flux state where you have so many turns to cover the unrest.