I sometimes like to think about the best ways to build up fluffy armies (mainly based upon the Horus Heresy novels) for the Traitor Légions in 40K. The Thousand Sons are maybe the most difficult to adapt because of a lot of gaps or incoherency between the codex and the fluff (or, maybe, the fluff as I understand it).
- Before being definitely turned towards Chaos, the TS were, for the majority, psykers, and their organization was matching the different types of powers existing within the Legion. Basically speaking, this looks close to the "Brotherhood of Psykers" concept (not available for CSM, though ... there is an impressive project of a pre-heresy TS based upon the Grey Knights codex here : http://www.warseer.com/forums/showth...ro-Project-Log
- Ahriman was a Corvidae, an Order specialized into Divination powers. Neither CSM, not Ahriman, have access to Divination (Tzeentch daemons do, however), apart from a Crimson Slaughter relic. The Telekinesis of the Raptora Cult has also ben wiped out from the 40K TS. Fluffy speaking, that could be explained by the fact that Tzeentch took those powers off the Legion, but I have never read something even close to that.
- The Rubric of Ahriman turned the less psychically powerful TS into Rubric Marines. Rubric Marines, by the rules, may not be fitted with Terminator Armour. So, fluffy speaking, it could mean that the whole Scarab Occult has been wiped out by the Wolves upon Prospero or that all its surviving members have not been affected by the Rubric (and are, therefore, only usable as Chaos Sorcerers in Terminator Armour).
- The same kind of reflexion may be extended to Jump Packs and Bikes, for instance (even if it could also mean that Rubric Marines have not been able to use such devices after transformation).
- It seems that the TS (or some of them) are somehow able to summon the souls of their dead brethen into mortal bodies, and also using their geneseed to create new TS. These are the only two (known, but rare) ways to line up 40K true non-rubric TS as usual SM, Havocs, Raptors, Terminators (for which specific bitz exist), Chosen, ... without the VoTLW (as all TS from that time are now Rubric Marines or IC Sorcerers).
- The 40K TS operate as small warbands seeking for Knowledge. That would cut off access to (large scale) daemon forges, as far as to huge bunchs of cultists (as they don't care about religion and worship), and these cults members are far more intellectuals than warriors. 40K TS are also Tzeentch's favoured agents into the Materium.
Considering all of the above (sorry for the long text), I would say that the most "fluffy" 40K Thousand Sons army list would consist in :
- The basics : one or two Sorcerers marked by Tzeentch (maybe with Pyromancy, which is a good complement to Tzeentch powers or Daemonology,), with a Spell Familiar (pre-heresy mentor spirit) and, maybe,the Book of Magnus, with two units of Rubric Marines in Rhinos, from the standar CSM or the Crimson Slaughter codex (if Divination is needed).
- Around those, some vehicles (Land Raider, Vindicator, ...) that could be "possessed" and ... Tzeentch daemons as allies (or, better, the TS warband being allied to a Tzeentch daemons army) : Screamers as bodyguards of the Sorcerer (if he stands upon a Disk of Tzeentch), Horrors to help with psychic powers, Lord of Change/Heralds (for Divination), Flamers (with Pyromancy discipline for the Sorcerers, making up one of the most "large scale burning" list of the game) or Chariots.
Something half way from a Screamerstar, a Daemons summoning or a "Let make it burn" list (depending on choices made about the psychic disciplines and size of the different units).
 I wrote that up while the previous answers were written too. Reading them, I agree with the "Rubric Terminators" (even if it has not been directly planned and integrated into the Codex), the use of Chaos Spawns (the final state of the TS degeneration) and the fact that some could have been away from the planet at the time of the casting of the Rubric of Ahriman.[/edit]