Cool! This is kind of related to an older question I asked about having a 2v1 boss fight gimmick. I'm not sure what your original intentions were with the futanari succubus character, but I was thinking if you were to redesign her and bring her back, she'd be an interesting boss to rework as a 2v1 boss. She could be paired with a futa angel and they'd be like a succubus and angel duo/ or devil and angel duo type boss.
They'd be from two opposing races and are supposed to be mortal enemies but instead love each other and work as pair/do everything together, including fighting enemies. They could also have contrasting personalities, such as one would be more bold and assertive, while the other is more shy and reserved.
If you wanted to include/were planning to include futa twin girls as well, you could also have them, but the twins would have a bitter rivalry instead and would compete for Nene affections. This could lead to a spitroast/ skewered fuck scene or something, with Nene in the middle.
This is just an idea though, so if you already have plans for a different futanari pair to work as a boss, that's fine as well.