Dark Eldar? Hmm, similar wishes as most here, although possibly more narrative-friendly : Better shooting at things in the air, without relying on fortifications. Just not narrative. The Razorwing isn't enough. So, probably just be able to buy Skyfire and (separately) Interceptors for Raiders and Ravagers.
Faster assault. First turn assault can be hugely disruptive, but the narrative suggests the DE of all the armies should have the best ability or possibility there. What I'd really like is having aethersails not count against combat speed ;-)
At least assaulting reserves out of the webway, dagnabit!!
Oh, a few survivability tweaks; 5++ for the Talos and Chronos (how is it narrative for a monstrous CREATED creature to not have an invuln?), ignore cover weapons are still disadvantaged against Reavers moving flat out, a racewide 5+ Deny the Witch (simply from their constant struggle against She Who Thirsts), I'd like a FAQ that the wyches' dodge save 'counts as' but is not an invuln, hence not affected by psychic effects, like psychic hood.
Really, relatively small things that simply emphasize the qualities the DE possess, None of these, except possibly assaulting with aethersails, change the game much. Webway? Still need a turn to get it in place, so it can't be set up ahead of time.
One other thing. I'd like mandrakes to be actually useful in the game. (and Kheragdruhkar as well). That would make every unit in the DE codex viable. And such gorgeous models.