Do you have an end in mind? I think, that's quite important, as it gives your creation-process a direction. Your end may change throughout your writing process, this is no problem. It's just to see the light at the end of a possibly long long tunnel. You may also determine an inciting event soon after the beginning and 2 major events at approximately 25% and 75% of your game.
The inciting incident is basically just a way to give your protagonist something to do at the beginning. Maybe it's the apocalypse, or just an event, where the player gets to know, that the protagonist struggles with her former life as a witch. Maybe her problem was, that her magic always ended in lewd acts why the other witches laughed about her and she may start to visit an self-help group for anonymous ex-witches. The first major event at the 25% mark of your game starts the actual plot and forces your witch-MC to act and make important decisions. This could be e.g. the apocalypse or the point where your witch meets other survivors and may be ripped out of some lethargy.
The second event major event would depend on how your game develops. Maybe throughout the second act, the witch tries to find a way to undo the apocalypse. Maybe she finds a way and is hesitant to be nude in public throughout the game but realises at 75% that she has to travel nude through time and is doomed to land in mid of a fully filled arena, what let's her become more comfortable with being nude in public/exhibitionism when she finally does.
Maybe she struggles continuously to use magic, as it always ends in lewd acts and only at the 75% mark, there is a major event that let's her realise, that magic is the only way for her to survive (and help others?) and that those lewd acts are nothing bad or sth. like that...
Maybe she makes some friends and only realises at the 75% mark, that one of them is actually an enemy, she finally has to defeat.
Maybe she makes some enemy throughout the game, she has to defeat, and loses an important fight at the 75% mark against them, but realises finally, what she needs to do to fight them or that she doesn't need to fight them but rather to reconcile.
...
Knowing those three events and the end (even if they change during the creation-process) will help you structure your creation and help you to go through your story without drifting too far away.
If you plan to make your game episodic, you can even use this method to make a plan for your whole game and additionally to structure every single episode inside the overarching plot.
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Regarding the kinks:
I personally would also suggest to initially focus on only a few kinks only what you know and what you like.
Gender-Bender could be interesting, but only if it fits good into the plot. If you just force it, it could get more silly than interesting.
You should also consider, that there are some kinks, that basically split the community.
You might e.g. find some fans of scat and diapers, but they are in a minority and you will also find a lot of persons who don't want to see it at all. It also may take you much time to create it but only give you few revenue, so think about if you even want to add sth. like that and when you introduce it, better keep it optional.
Watersports is similar, but here you might find more fans and more people for whom this is acceptable, but better keep it optional as well.
Kinks that probably completely split the community are loli and rape. There are in general some fans (but I'm actually not sure, if those would even be interested in a game with bisexual-content), but for me and also many other people those would be a very hard pass, even if they are optional.
If you try to monetize your game, you also should avoid any incest. This has a huge fanbase but is a nogo for Patreon.
