Hello @Arandomquitter, the most difficult is to think about the main mechanic of your game.
Because all game is going to be based on it, for example:
A game where the character can climb using a tool… all the environment is prepared for that…
All the skills, animations, enemies, … are going to be ready for that.
Think about Resident Evil, Alan Wake, Dead Space, Silent Hill… all are different. So tell me, what makes different your game?