Cannot say much about 3 yet; just a few hours in. But it makes a good first impression.

GalCiv 2 ultimate, while different to MoO2, is the one Space 4X I spend at least as much if not more time in it than in MoO2. Its biggest weakness to MoO is the combat which you cannot control directly. When you attack (or get attacked) another ship or fleet its automatic; you can watch it in a kind or cinematic sequence in which the ships duck it out in real-time or get the results as quick combat.

Tech tree is more diverse; also there are several tech trees for different races which requires you to either steal or negotiate for some other racial tech if you want it (no requirement to beat a session). Some techs get unlocked by events (i.e. galactic plague; be the first to research the cure and be either the savior of the universe or a total ass about it ;) ).

Diplomacy: Its fun and its worth it trying out. The AI races act as you expect; with the honorable keeping up being honorable even if you are weaker (unlike MoOs pacifist Psilons which burn the galaxy in a genocidal crusade as soon as they can) & they act more accordingly to what you do. I remember on GalCiv session in which one of the AI ambassador called me at one turn and asked me if I think they are stupid which was actually a valid question since I did build up fleets near them planning an invasion.

Galactic Mega Events: They can mess you up. But they can also mess the AI up. Its not a few turns on inconvenience; its shit happend: change any plans you had because this changes everything. (its always handy to have a save close to before such an event ;) ).

Anomalies: random re-spawning space anomalies which can add a tiny empire wide boost to one department (like +1% ships weapons) or just teleport your ship to another place on the map. If you just play quick games on small maps they are pretty much useless to you, but if you like to play single sessions as long as possible those +1 really add up over time. I remember one really late game mega event with Paramilitary Indominable Raging Alien Threat from Extradimensional Space which usually kill everything of the map, but with all the boni collected over time it was one of the rare cases my ships where actually able to defeat them.