I'd love to see the developing relationship between Zannah and Cognus... and the various disagreements and fights between the two.
Zannah is first and formost a Sorceress and Scholar, hungry for knowledge, while Cognus is much more of a warrior... so I imagine an almost Revan/Malak-like relationship between the two.
I'd like to see them explore various former sith worlds collecting lost sith techniques and knowledge, I think Ziost and Drommund Kaas would be great places to start.
Also, this is very much still a transitionary period for the Sith... Cognus would be one to strike first and ask question later, while Zannah would obsessively overanaliyze a situation to the point of Cognus' constant infuriation, so plenty of fights, some of them even devolving into violence.
Cognus meanwhile has her own strengths... she's conniving and manipulative along with being very unconventional... she'll always be looking for ways to undermine the Republic and Jedi in anyway possible. So maybe she sparks a bloody war somewhere?
What do you think of these ideas?