God-like or Ancient Races (like Babylon 5) would be interesting and could be fun. The idea of a powerful entity or group in the game that did not have the same objectives (colonization) as the other races/groups in the Game. A race of beings (or several like the Vorlons and the Shadows) that were very powerful, damn near invincible, and they fought each other, searched for specific things, and that resulted in the destruction of colonies or ships. And they would never explain themselves or their reasoning. Hell even from time to time (very infrequently or maybe this could be a scenario) they could just clean house wiping out 90% of the galaxies colonies or ships, or maybe just an empire they decided that didn't need to be there anymore.
To add, maybe, to their difficulty or mystery they would mostly ignore attempts to be communicated with.
I am a huge 4x game fan. As always I am up for a discussion, even a passionate one. If you are polite to me I am polite to you. If you are rude to me you will be ignored. After all its the internet and we don't know each other.