Awesome. I toyed with the idea of trying to get a Dark Eldar army together for quite some time and ended up deciding I have enough on my plate right now. That's cool news for me.
From where I sit, there will be two challenges in getting them into DoW. The first is they're extremely fast and extremely fragile. Like, they can move across the board in a turn, but if they hit a pothole on the way they'll all die. That's harder too pull off in real time because you can't have things move too fast or die too quickly because it just never works. They don't really build up like a traditional army since they're primarily raiders, so they tend to hit hard then run off before anyone really has a chance to fire back. Relic will have to take some liberties, but it's not like this is new stuff to them and they've done well so far so that'll probably be fine.
The real problem will be just the look of the Dark Eldar. The line of models is probably the oldest in collection and a lot of them are in complete need of an update to look on par with the other races. They have a few cool looking guys (the standard Warrior is pretty sweet), but Relic needs to put their stamp on a few of them to bring them up to date. That could also be the start of something else for Warhammer in general as well. Since the Dark Eldar line are one of the oldest in circulation and usually one of the first cited as needing an update, this could be the hint that'll happen within the next year or so. The last expansion lined up with the update to the normal Eldar and sorta helped confirm that the Harlequins were getting an update, so maybe this is something similar.