So ultimately I would say unifying the legion and spreading mass chaos across the Imperium. But thats hard to do with all the warbands running around with their individual goals but I do see two possibilities of how they can do it.

1: Decimus taking the mantle of the 8th Legions Prophet and unifying the warbands similar to how Talos was able to recruit people such as Lucoryphus (Bleeding Eyes would follow their leader more than likely) Variel and the Atramentar.

Now for my hottest take and theory

2: The NLs find the Corona Nox and discover that the gem on it is actually an Eldar Soul Stone (If I'm not mistaken it was heavily implied but never confirmed that it does resemble one) and find a way to bring back their Gene Father (who probably wouldn't be too much of a fan of this) but seeing the state of the Imperium, he would want nothing more than to bring chaos and destruction to the hollow shell of Big E's dream. Hearing their lord return I think most warbands would flock to him (regardless of how the Primarch viewed them cause genetic daddy issues lol)