Genre mash-up modding possibilities abound!
- Space trader, c.f. Lavamind's Gazilliionaire. You build a Trader ship, and race other empires' traders to buy cargos cheap and sell them dear. It's almost RPG-like, with your ship as a "character".
- University student. An RPG-like setting, where you role-play a Student, and you go through Schools to gain Skills. I made up this niche as a totally different Thing To Do in the context of a galactic empire at peace (i.e. when I got tired of the space trading niche).
- Rock star? Maybe you're a band, and you travel from gig to gig. Could dovetail with GC3 tourism improvements, e.g. you hope someday to sell out a Stadium.
It'd be funny as heck to twist GC3 into a peacetime game-within-a-game. Wars would be punctuation that temporarily interrupt service ...
That reminds me of those scenarios in Civ 3(?), where you played the hero going from town to town and saving it/doing stuff/meeting the local ladies 
Interesting concept for a mod though, I can see the Drengi fleet chasing after your rock band because their metal concert wasn't brutal enough
Benevolent: You host a charity rock concert to benefit starving Stardock Developers.
+10 Benevolence, -50 Credits, increased attendance at your next concert by 5.
Pragmatic: You put on a show that caters to your local audience tastes'.
+10 Pragmatism, +30% Revenue from concert, and:
[IF Altarian Audience] Hitting those angelic high notes causes your throat to be sore, unable to perform another concert for 3 turns.
[IF Human Audience] Humans love to complain about everything, lose 10% Popularity for 3 turns.
[IF Drengi Audience] One of your band members lost a limb because they catered to a Drengi's taste quite literally, he will recover in hospital for 3 turns.
Malevolent: You rent a bunch of bats and all band members eat their heads on stage.
+10 Malevolence, +30% Revenue from concert, +15% Popularity, Benevolent civilizations ban your concerts for 10 turns.