About that, You talked about Sylvia and thought about including her in your game. If you wish so I had 2 ideas for a bad and good ending that would stay true to her character.
In the good ending NON NTR, Kazuma manages to ally himself with Sylvia. As a compensation for his betrayal, he would give her John as a sex slave. She would get to peg the prideful Jon and break him for all of eternity while Kazuma would live happily ever after with his 4 girls.
In the bad NTR ending, the opposite happens, and John is the one who does the deal with Sylvia. She would get to keep Kazuma all to herself to satisfy her lust and her affection for him (and also getting revenge at Kazuma for his betrayal) while John would get to keep the girls for his harem.
Another suggestion if you want is that since the beginning of the game, Wiz NTR scene have been very isolated, perhaps it would be good if she would be more “included” into John’s harem and she would get to have scenes of complicity with Darkness, Aqua or Megumin.
Also, I was wondering, are you thinking about including more girls from Konosuba. Luna or Suna would be a welcome addition for both the vanilla and the NTR roads, there is a lot of potential there.
In the end, it’s up to you choose whatever you want, if you want to keep those ideas, put them in bank or not use them at all, it’s really up to you. Do with them as you will.