I've had a couple of ideas for fun allies lists, post your own ideas!
-A combined Ordo Xenos radical and Eldar force. Farseer and bodyguard working with a radical Ordo Xenos Inquisitor represented by Imperial Guard.
-Grey Knight-Eldar Daemon strike force. Inspired from the bit in the GK codex where the Grey Knights send for the Eldar to secure the spirit stones of Malan'tai to prevent another daemon incursion. Indicates some level of cooperation between the two, this list takes it to a new level, joining forces to fight off daemonic incurions before they can spread.
-Craftworld Eldar/Corsair list from IA11.
-Dark Eldar/Imperial Guard. Dark Eldar raiding force accompanies by human slaves and mercenaries.
-Dark Eldar/Tyranids. Wych cult using a variety of wild beasts, uses a mix of tyranid models and other beasties to represent the 'nids.