Here's an idea I got re-reading this and other threads. Can be blended with other ideas pretty easily.
For now I'm calling this race the Xeno
('cause I'm being lazy on that front). Suppose they embodied an issue we're facing today? Make of them a metaphor regarding our own lives, the issues we're facing right this second?
The Xeno are a people with a long history, much older than that of Earth or Vulcan. After developing warp drive, they expanded outward. Another race--let us call them Alphans
--were in their way. A war of conquest was long and bloody, with many Alphan refugees fleeing the area as the Xeno Imperium grew. Again, all this was a long time ago, maybe a thousand years. The Imperium wasn't a uniformly bad thing, but it held onto its territories brutally. It eventually let Alphans into its civil service and military. But over time, a series of civil wars and rebellions grew into catastrophe. Whole worlds were made uninhabitable. The Xeno were forced back to their home system where they began the long effort to undo centuries of ecological devastation there.
Today, the Xeno have a very mixed reputation. Some look back upon their Imperium with great fondness, longing for past glories. This has given rise to an Imperial Faction
perfectly willing, even eager, to kill and destroy towards their ends. Most Xeno are just ordinary folk, albeit with a lot of pride in their past and slightly condescending attitudes towards other races. They have a fearsome reputation but little justification for it, not anymore. In fact the so-called Modern Faction
is very suspicious of anything that hints of taking up their old imperialist ways. One reason they joined the Federation was to avoid having a military of their own!
In short, a civilization with some real darkness as well as glory in their past, one in which they feel pride as well as shame. Others routinely mis-judge them, in one way or another, while they themselves continue to come to terms with issues still not fully resolved. Call them an analogy for the modern day Germans, for Arabic Muslims, for Americans still trying to come to terms with slavery, or whatever.
Naturally, having either an Alphan or a Xeno (or both) as a regular would be the way to make this work dramatically.