Seeing the type of game i will give some recommendations:
-First of all a captive system that has effects on the character, if you are defeated by an enemy it shouldn't be game over but be captured and brainwashed by the enemy.
*It should be progresive, the more you are defeated and the longer you are captured and fucked and the less you will resist when fighting that certain type of enemy. When 100% brainwash you would have one of the following "insta-lose or a bad ending".
*A lot of enemies wouldn't have that captive system because of the type they are, in that case you will be violated and get a debuff for a time.
-Second is corruption, if it's a dark world i think it's a must. This should increase slowly by being defeated or by certain actions (if available).
*What type of actions?, well the usual ones, exhibitionism and any you may think.
*It should increase just a bit by being defeated and violated.
-Third is giving a bit of love to the town. I would recommend a few relationships that you can have in town and some bad events. However if you want to maintain that "dark" mechanic the relationships should be... a bit twisted. For example a cute girl that is with you just so that se can get your money, a femdom girl or things like that. About the bad events just let the city have slavery and add a slum part, the rest is easy, just think about what you truly want to make and the effects it should have.
-About transformations... it's not entirely necessary, however if you add them be aware of the hard work it will suppose, if you have cgs it will mean a variation for every cg that you can access. If you add it be aware of it, i recommend that if you wanna use it then only for bad endings.
-Dungeons will be the focus on the game, so it's important that there is some "diversity" between dungeons, some enemys should be unique for a certain type of dungeon.
-Another thing and this just would be great if you could it's to add "titles" depending on your adventure. Specially if you are defeated by a certain monster, or things like that.
-Lastly a few recommendations about combat system. It would be great if you can make the character stronger by fighting, for example if the player use mana, at the end of the combat he will receive a little (or considerable depending on how mana did he use) increase, and not by leveling up, in fact if you want to be near a dark souls game i would recommend that you will find objects in the dungeons that will give you skills, however if you want that part to be perfect, make some requirements for learning some skills.
I will add how difficult would be to do some of the things i have said:
-Captive system: Normal difficulty, it would require to create a new stat that symbolises it. About the brainwash effect it would be mostly the same, but adding debuff and a few effects depending on how much it advances.
-Corruption: Hard difficulty, it's hard because it's one of the things that has most of the effects, a good corruption system will change mostly all conversations.
-Slavery system: Hard difficulty, because of the same reason as corruption, it requires a LOT of work.
-Transformations: There is quite a big possibility that adding this will create a lot of bugs, disparities, etc... Difficulty VERY hard, as i said i recommend that only in bad endings you add it so it's for a few scenes at most.
-Dungeons diversity: Easy difficulty, it's just adding different types of enemies to the different dungeons. It would be good if every dungeon had it's own charisma.
-Titles: Easy-Hard difficulty, depending if it's something esthetic or if it will have effects on the game (giving or taking stats or affecting conversations).
-Combat system: Very hard, it would mean modify the combat of the game a bit, which can be perhaps too hard for some.
Lastly just to say, it's quite important to have a bit of lore, some tragic stories and etc... just so it helps to give the perfect atmosphere, and a bit of main story that explains what we want to do and why.
This are all the recommendations i can give, feel free to ignore them and good luck with your game.
