Last one that hasn't be talked about is the first one I ever finished, the Yuhts.
After over a million of year believing they were the only sentient species in the galaxy, they finally encountered another one. After the shock had passed, they realized that, actually, they liked being the only sentient species, and promptly tried the wipe them out. One tiny problem though; while the other species had been around for a tiny fraction of the time the Yuhts had, this other species had developed several technologies - including FTL travel - that the Yuhts had never even thought of, giving them a decisive advantage that eventually led to them wiping out the Yuhts. Yuhts must have never read The Road Less Traveled.
I kinda hope they expand on these, especially now that we are all but certain to finish the chain now. Not only new precursors, but also unique bonuses to each on. ATM the only one that offers something good beyond a system you can't colonize (well, as of 1.4) and a bunch of science is the Cybrex with Living Metal (which isn't really unique, just a guaranteed source of Living Metal in the galaxy that your empire is going to be close to a source of). I mean, the Yuhts lived for thousands of years and were colonizing space with slower-than-light travel, the Vultam made VR that was so realistic that they began to doubt reality, the Irrassians apparently had some pretty nifty medical tech to compensate for their poor immune system and the First League kept up a cosmopolitan empire/federation with a planet-sized capital city for thousands of years. Surely they have something that your Empire can use that it might have never discovered otherwise.