We will be the salvation and guide for humanity after we cleanse the galaxy of the sins of the grand father.
All is Dust, after all. And if it is not now, it will be soon. Especially the corrupt and bloated "government" known as the Imperium of Man. For Tzeentch and Chaos!
It depends on how they want to proceed with the factions, but if they want to follow lore and put in some chaos gods into the twist as a core point for the factions, so they get pushed out there to play the lore part, Thousand sons is the best faction for Tzeentch followers.
I know that including the Thousand Sons into a game like Eternal Crusade can be quite difficult from a lore standpoint - but still, I would absolutely adore it if the Thousand Sons were playable in this, seeing how they are my single most favorite legion in all of WH40k.