In my opinion, being a 40k fan for a long while now, the Eldar if they ever get a core book, would need a THOROUGH background section. I don't mean just craftworlds or history etc. I mean a comprehensive section on their mindset and how they think. For those players who want to roleplay Eldar, me slightly included, we'd need a really really firm idea as to how different the Eldar from us.(general fluff both new and old), They have the attention spans of a knat at times. Those locked on their path are those who don Aspect Warrior Exarch armor, the Farseers, Autarchs to name a few. An eldar citizen may go on a 100 year bout of being an Aspect warrior, and later get distracted by something and go study music. Though they share common feelings and certain facets with humanity in the end their minds are different from birth, from humans. This is a race that is dying out, and they know it. Humanity in 40k is blind to this ultimate truth, they dnt except it. Eldar however know they are dieing. These few reasons listed is why if an Eldar book is to be created it needs to have a comprehensive section to help players and the gm understand HOW different the eldar are from humans in 40k, or in general. Other than that the mindset of the different existing craftworlds in general should be gone into detail as to how a character from that wolrd feels. In closing if an Eldar books is to be done it needs a thorough section on how fundamentaly, psychologicaly, and morally the Eldar are different from humanity, or in my opinion it will fail. Cheers!
P.S. Bump on the possibility of an Ork core book, their cunning and brutality would be a real fun game to roleplay WAAAAAAAGHH