So doing a little Google I found the rules for dark eldar apocalypse formations, looking at some of them, they could actually fit in regular games of 40k without people crying cheese. Titan hunters lets you take a squadron of ravagers which gain shrouded, carnival of pain, dark Olympian and kabalite web strike are all pretty decent as well. The Valedor one is pretty decent as well, so it wouldn't be too much to ask to allow these formations in regular games would it?