Greetings,
I've been playing tabletop 40K for years. So of course this sparks my interest. Quite frankly I would be more than willing to play a role under the Chaos Space Marines, the Tau, or the Eldar. However, I am much better versed with the CSM and the Tau as opposed to the Eldar.
On another note, one cannot be a Sorcerer for the thousand sons if they are trying to play Chaos. Chaos and Chaos Space Marines (which host the Thousand Sons) are two separate factions that work independently of each other and quite frankly the only reason the CSM are not entirely devoured by the Chaos is because of their continued devotion and successful actions in combat. But quite frankly, the gods of Chaos would easily assimilate the CSM if the need arose.
On any accord, you said Chaos Gods were out of the question. And it sounds as though you don't want cannon independent characters either. But let's assume I play a role under the Chaos Space Marines or the Tau, would you be opposed to a Daemon Prince or an XV8/9 Crisis Suit Commander?
Might I also recommend, if those involved are so well versed and if it should play a substantial role to subject the participants to a point requirement to maintain a suitable level of power? Because off the top of my head, I could easily fill a role and make it so intense that as a single player it would easily obliterate the masses.
Just some thoughts,