Civil War

Put your tax rate to 100% and leave it there.

Your civ will eventually split in a bloodless civil war. They make a new minor race. Blood will be spilled bringing the rogue planets back under control.

Hehehe

I actually did that (See the AAR "Green Pasture"). It only took one turn to lose 30 some of a 80-100 planet empire. It only took four or five turns to get them all back, though - but not without consequences.

I think a civil war mega event would be great would help level the playing field when a civ gets really powerful. But it needs to be a mega event in the truest sense. Civil wars aren't pretty and devastate societies. Would require something like 25-50% of planets and equal percentage of fleets rebel and become new minor race (or maybe a major if that's possible). Could have several possible triggers each with a different probability.

Possible triggers:
1) long duration of low morale or excessive taxe rate. for example, say greater than 50% tax rate. Sure you can abuse morale buildings to have 80% tax rates and high approval, but who in world would really be happy handing over 80% of their paycheck??. (Highest probability trigger. Even higher probabability if your party loses majority in senate.)

2) Excessive warmongering. Citizens get tired of you declaring one war after another another (probability would start low but incrase as # of surviving major races decreases).

3) No other races share your moral view. eg: One evil race and three good. Maybe your people start to question who's "right". (lowest probability as it would require all races to have formally picked an alignment first).

In addition, the new race should start out at war with the civ it broke off from.

Yeah, such an event would be pretty cool. Keeps you a reason to monitor the overall happiness of your empire, lest you want it to fall apart or split.

In addition, a good idea could be the ability to actively support the splinter group from the original empire.

Was this already suggested in the thread where Frogboy asked the community what they would like to see added in future updates? Does anybody know?

or you could cause a rebellion in an another civ with spies

I believe you could do this in the old Master of Orion series. Would be an interesting tactic.

There should probaly be a choice, because there always is