So I really would love to lose a few sectors of my theoretical empire to something sadistically horrible, like a plague with no cure, massive civil war, 20+ civs declaring war, or ideology changing and people hate you, or all the above its all fine as long as it makes me cry out of frustration and fear.
Meh, sounds a bit weak. 
GalCiv 2 had a much worse one: Sub-Space Tear. It permanently limited your max speed to 5pc per week. Imagine having this happen to you on a "truly insanely large galaxy" (as Frogboy put it). Your enemy is 1000 hexes away from you? That will be 200 turns before you can reach them. Have fun! 
On a more serious note, Mega Events are supposed to be unbalancing. However, I hope, that the ones in GalCiv 3 won't be so OP, that they push you to rage-quit. I could handle pretty much all of the MEs in GalCiv 2, but the Sub-Space Tear always brought me close to quitting the game, because I prefer to play on immense maps. To put it bluntly, I'd rather have the Jagged Knife steal half my empire ten games in a row, than experience the Sub-Space Tear even once. 
