Just realized my game idea could be construed as a combo of Emperor of the Fading Suns, Crusader Kings 2, Hearts of Iron 3, Sword of the Stars 2 and a dash of Chapter Master.
Moar!:
The Eldar and Dark Eldar are 2 sides of a similar coin: both are tied to the webway to stabilize their realms. For Eldar it's very similar to the Imperium for the most part, but way smaller scale.
Expanding is as simple as raising a scout team with a bonesinger team attached and putting them on a fast scout ship. Get them to a planet and get cracking on a Webway gate asap. Once it's linked in you can begin building normally, and population will slowly flood in. Slow and stable; that's the Eldar way. Once they put up a webway gate, they're almost impossible (or not) to dislodge, but their growth rate is horrible. Don't expect many fast expansion campaigns like the Imps and the Orkz. While we're at it, your military is very powerful based on its composition. Their unit roster, like in the war games, is very specialized. Send the right composition for the right enemy; with the webway system linking all your planets, you can quickly edit an army and watch them reorganize and warp somewhere quick. As you aren't as separated as the other races, you have alot more control over your planets then the Imperials and the like.
Dark Eldar are very similar, but with the inclusion of Slavery. In this way they have 2 populations per planet: Slaves (any sentient race, mostly human tho, they don't care) and Eldar. While the Eldar have the bonesingers to take care of most constructions in a very civilized manner, the Dark Eldar whip dem slaves good to get anything done. As such, you'll find yourself in a very similar position as the Orkz; gotta raid those lesser races to keep slave production up as you'll be burning through them constantly. Run out, and nothing will get built with any speed. Also, as in all chaos factions, you can choose to follow any of the Chaos gods for different bonuses. Khorne if you wanna bathe the galaxy in the blood of your enemies, Slaaneshi to get that population up and to speed conversion rates on captured planets, Nurgle to help keep slaves lasting longer and causes strong attrition damage on all of your worlds in case you get invaded, and Tzeentch for a plethora of small military bonuses and an increase in your commanders skills. You start out as Chaos Undivided with very little bonuses but no drawbacks for not sacrificing slaves/enemies/fellow eldar, and it will eventually behoove you to pick a side; but beware to choose the one you'll stick with, the Chaos gods don't like flip floppers.
....The Eldar play too much like the Hivers, plz no sue me.