Hello novice dev!. First of all I would like to give you a guide about "how to develop a game" so that you can have a good start:
1º Define the world, lore and the main situation/conflict: Overall we are talking about an isekai game, so it's important to talk about what's the conflict, why did he get invoked and the situation that is developing there. Even more talking about the races you may encounter and their personality is quite useful, because it's an important part of the lore.
2º Define the MC and the heroines: Overall about the MC you have to think if you wanna add just combat stats or skills that can be used out of combat like persuasion, handsomeness... depending on the tone of the game and the situations could be required sex stats and even a sex skill stat. About the heroines you have to make their basic personality, give them a "life" in the sense of who they are, family (if the have), personality and finally sex kinks.
3º Define fetishes of the game, divide them in main fetishes and secondary ones, main fetishes represent the target of your public so they need some dedicated implementation. It isn't the same a game that focuses on femdom than a game that focuses on maledom, actions and how the game is should be taken into account. Overall secondary fetishes aren't integrated into the game but rather are an extra that may happen in some sex scenes based on the player's choice.
4º Prepare the game so that everything you want to do is easy to implement: This requires quite a bit of coding, overall this step will make it easy to add whatever you want and give you quite a lot of flexibility, the easiest way that developers follow when making a game is that first "event A happens", then "event B happens after event A" and continue, with a good coding or preparing you could make random events and events not only based on random things but conditions, "event happens if corruption > 40" this is the use for character stats, because later they can be used as conditions, even more an advanced game may add traits and use even the existence of that trait as a condition.
5º Write all quests you are gonna add before implementing them into the game, having a general idea and trying to implement it at the same time you think about it can give a really weird result, take always your time before implementing it, doing thing fast can end in a big mess and many bugs. Even more, taking your time with the coding at the start and preparing the game for what you wanna do will make it so that you don't have to review or add things in the future, which will help a lot in the stability.
6º Take care of details, things like changing the letter type of the menus, giving a bit of colour and customizing it a bit may be a bit of work at first, but the presentation of the game will be much better than if you leave the default ones and it will attract more public.
7º If you wanna attract people to your game, you need to sell the game, the introduction, fetishes and images must be used in a starting post, even more they should be displayed in a way that the possible player understands what they can do in the game.
Overall those 7 are my tiny guide of "what every developer should do when starting", taking your time at the start to prepare everything is a must for a successful development. Many games end up having big problems because of being lax at the start, having to change mechanics and then the game goes wild with bugs. This is what generates the typical situation of "I need to change game engine/ I need to remake the game". In many times it has resulted in the death of a game, so be aware of it.
