If we're just spitballing out ideas, in that scenario I'd rather try to ally with the nemesis. I doubt they're being treated particularly well by their overlord(s)
Based on the unused CG and the idea of she being a
nemesis, I envisioned the nemesis character being extremely corrupted, she has already fully embraced the idea that "might makes right" to the point where she doesn't even resent her overlord anymore, and at least at first she wouldn't have any reason to think allying Rowan would give her any benefits (maybe she could fakeout sympathizing with Rowan's position to get the best of him early on).
Maybe as a conclusion to their rivalry she could be made to switch sides if Rowan is the victor: A corrupt Rowan could convince her he is "dark overlord" material and that she should serve him. And a Rowan that is loyal to the Twins could convince her to join the winning side and betray her overlord, with both of them serving the Twins now.
But for a pure Rowan? I think she would hate his guts and have massive resentment over the fact he still thinks he can be a real hero after being made to serve demons. I don't think there should be any way of turning her unless we go the route of Rowan somehow managing to "purify" her and getting her to restore the good in her that had been buried down for so long... of course that, knowing this game, imediatelly afterwards she would be caught and Rowan would be forced to watch while she was tortured and mindbroken.
Same issue why the system was put to the side: too much work while they still need to progress the story and make the basic systems working well.
I get the feeling the nemesis system won't return anytime soon, if it returns.
I said it would be an interesting idea by the time act 2 rolls around, we're still probably over a year from that since the devs are still gonna finish revamping the skills and equipment systems, add the entire goblin forest area and quest, rework the conquest of Rastedel, and who knows what else before they consider act 1 finished. I don't think it would be too unreasonable to expect the devs to have the "basic systems working well" by then, and if that's the case then I think the nemesis system would be a pretty cool way of expanding beyond the basic systems.
Ultimately, I want the nemesis system for act 2 particularly because by then the player needs a break from having the Twins as constant antagonists they can't overcome. For a corrupted Rowan, regardless if he is truly loyal to them or not, it wouldn't even make sense for them to keep acting so extremely antagonistic to him since Rowan is becoming everything they wanted him to be. And for a pure Rowan, since he probably won't get the best out of the Twins until act 3, might as well give him an antagonist he is allowed to beat for act 2, because like I said before, it would be extremely tiresome if act 2 continued with Rowan unable to get any real victories.