I don't have any insider info about what the devs have planned for the future, but I think the nemesis could make a lot of sense for act 2.
With the Twins having a target on their backs after conquering a city as large as Rastedel, they will probably have to seriously look into alliances with other corrupted forces, the nemesis could potentially be a liaison of one of the other big players sent to the castle to coordinate their efforts. Bonus points if she's a former hero like Rowan or has been building a heroic reputation so that it also makes sense for them to have to keep a friendly appearance and occasionally work together when spying on the humans.
I think such a dynamic could lead to interesting interactions: If Rowan is becoming corrupt then their rivarly could be instigated by their respective masters pitting them against each other to see who was the best "corrupted champion". If Rowan is submissive to the Twins then they could try to command him to best the nemesis while they continue using and humiliating him, to intimidate the nemesis and send a message ("we can enslave one of the most competent heroes and turn him into our bitch, imagine what we could do to you"). And if Rowan is still defying the Twins and resisting the corruption, then the nemesis could REALLY have it out for him, and the Twins could give her carte blanche to fuck with Rowan as much as she wants if it means breaking his spirit and having him give up on any kind of heroism.