But in GalCiv III, the events are XML driven (ala Elemental). Heck, that also means modders can add their own events and it means that events can have a severity scale which means they can be hooked up to a setting UI.
I'm very glad to hear this as I really like the Elemental events. In particular, I like the plague one where there are three solutions, at least one of which spawns a quest. More ways for the player to engage with the event and take different aproaches are always good.
I also think that the event that spawns skeleton armies across the map is a much better take on the Mega-Pirates/Peacekeepers sort of event - the skeletons are strong-ish but still manageable when they spawn, but those you don't kill quickly tend to mob up with other monsters and stay a threat. I prefer this to the "everything gets destroyed and you have to turtle for a decade or two" nature of the GalCiv II events, which is fun once or twice but can also be just annoying. Those that spawn a single powerful monster are also neat and add a twist to the game without derailing it; I think they would translate to GalCiv fairly easily (e.g. giant trogg smashing things with a giant sword becomes space hulk full of ravening aliens heading for one of your worlds).
Favourite Events: Aside from the Elemental ones mentioned above, the Dreadlords, the plague and the telenanth for ones that add a twist, and the new faction type ones for potential game-derailers (with the JK being sometimes gamebreaking but also often quite fun). The assassination/war event. Also the deceptively small things like changes of alignment and discovering new ships.
Least Favourite Events: The slowdown (on larger maps), Mega-Pirates and Peacekeepers (almost always). All these could be implemented in such a way as to be actually fun, however. Spypocalypse, which is just horrible.
Events I'd Like to See:
Discovery of a stable wormhole between two distant empires (grew up on Deep Space Nine) and/or a region of space becoming harder to cross - either way, suddenly it's easier or harder to reach your friends/allies. Particularly if the diplomatic relations system is sophisticated enough to take into account that suddenly the Federation and Dominion are neighbours, and the AI appreciates the strategic importance of a shortcut.
New major race shows up. A proper major race who's on more-or-less equal footing in terms of AI capabilities.
The privateers: A privateer captain who will (for a price) harass enemy ships - but who can just as easily change sides.
The extremists: A section of your fleet mutinies and attacks a enemy with whom you have poor relations. Do you hunt them down (e.g. seek and destroy ships with the capabilities of your own fleets) or support them and officially declare war? Or prevaricate and risk the other empire launching retaliatory strikes? From what I read about the background of GalCiv III, there may be an element of this in the campaign, it would be cool to see it in sandbox games too.
The Second Foundation: The Drath weren't wiped out, they were just hiding. Now they're steering events to their own ends from behind the scenes. You can try to get on with things, or you can seek out their new hiding place - but if you do the latter, be assured they'll do all they can to stop you.
Not sure how practical all these suggestions are, but I don'e think any would be impossible to code in some form, if not necessarily exactly as described here.
But what's the possibility for events that give side-missions?
Hopefully very likely - there's already a good system in place in Elemental, just replace "an old inn" with "a seedy cantina" and "old ruins" with "an abandoned space station" and have them pop up from time to time.