From the modelling opportunities provided within the Pre-Heresy time frame, to making the elite units stand out, eg. Catulan Reavers & the Justaerin Terminators, with different modelling and their own differing armour colours.
You can also, due to the in depth coverage of the legion, model any number of "named" Characters within the legion to lead your own force, or go all out and model the entire Mournival.
But one of the other great things in terms of flexibility, throughout the Legions history they have fought next to just about every other legion and organisation of the Imperium, giving you the chance to model something different, and even add splashes of colour to what is normally a very Black & White Force.
You can field units of Custodian Guard, Sisters of Silence, Mechanicus robots and even the vast units of the Imperial Army, to add a little diversity.
With this in mind, and needing a break from painting white power armour. I first added units of Emperor's Children to represent my Sternguard Veteran squads, followed by my take on Ancient Rylanor for their direct support. Having recently read Graham McNeill's "A Thousand Sons", and reading of how Magnus, sent members of his legion to other legions to start their Librarian Corps, it inspired me to add something that had been lacking from my legion organisation, due to fluff constraints at the time. A librarian.
So I present to you all,
Brother "Anonymous", of the Pavoni Sect of the Thousand Sons Legion.
- Currently attached to the Luna Wolves.
I hope you like him and think of ways to add diversity to your own forces, within the Fluff, instead of in spite of it.