I think some of these races would be fairly easy to modify.
Hopfully at some point GW delivers a cultist/stealer army/rules that could tackle both those at once.
Eldar of the RT era are basically Craftworld Raiding Parties/Raiders until 2nd edition so basically minus Wraith-anything/Aspects plus raider style jump/winged troops and maybe plus some sort of unique predecessor flyer/tank-s
Orks would minus some tech, plus more psykers, and My personal favorite missing element Digganobs! Perhaps some of the brood brother level cultist models +Tattooooos lots of em.
Im willing to accept the Tau were off minding their own buisness, and that necrons tombs were encountered similar to as they are now