I'd like to make an open-ended scenario where you don't have a clear objective as such but instead have a dozen possibilites and options, sort of like you had in Battlecruiser Millenium.
Your objective could be something open, like to get 20 000 points or something. You could do this by building a huge city, destroying enemies, exploring, and so on. I'd add elements like mercenaries, pirates, open side-quests, a branching storyline...
Woad Creations veteran, WiC junkie