Hi, I don't know if you're open to future opinions, but I think avoidable NTR (meaning there's a way to completely avoid it, meaning they don't even touch the girl) is much better than avoidable NTR because it gives more intrigue than knowing it's going to be inevitable.
Otherwise, you already know what's going to happen. It's also better because by being able to avoid NTR, you can also create the vanilla route and have both in one. For example, (the player wants to create the vanilla harem ending, or they want Aqua to have NTR, dark NTR, and vanilla Megumin).
Oh, and as for Megumin, I don't know if she'll have a scene, but if she does, then Megumin should hate whoever does NTR to her because, well, Megumin loves Kazuma and in the novel they're boyfriend and girlfriend and all.
And they could also create Kazuma NTRs, meaning Kazuma has sex with another girl or is kidnapped by an evil succubus, instead of having sex with someone in the group.
But like I said, these are just opinions; you decide what you're going to do.
I also really liked the game and if you like any idea then you can use it.