From my point of view, the minimum you must have is a story.
Not just what you wrote here but a real story with a beginning a middle and an end. And in your head it's not enough ... you have to write it. Not dialogues but rather ... step 1, step 2, step 3 etc ... until the end.
Your story would also need a past, well thought out. Even if no player will be aware of all the past that you have imagined the fact of having imagined one can use it if necessary to describe and explain certain things.
The writing of a story requires a lot of steps that you may already know (the first idea, the theme, the main character and his quest, obstacles to his quest, other characters ...)
From there ... It gets complicated. You have to offer interactivity, otherwise it would not be really a game. If it's linear with choices to vary some scene. Or something with several paths and therefore think of an alternative to your basic story.
Please do not put bad end screen. If you come to a point in your story and you say that there is a choice to be made but you are not ready to produce the different possible alternatives then consider that there is no choice. Explain why the character makes this unique choice and moves forward.
You are not there to educate people about what is a good choice or not, you are hear to offer a story.
Obviously if one of the possibilities is death then yes you can offer a choice that ends the story but produces, at least a little, this idea of death of the character and its consequences.
And then, you can consider the production (code, rendering / drawings etc ...)
If you go into the idea of working with people outside (designer, coder ...) then you also need a summary of your story to be able to give them and allow them to see if it interests them and if they can produce it .
A bit like a writer who wants to make a film, he looks for producers and shows them his idea.
Some may want a little more, for example how you write dialogues to see if it suits them etc .. but it will be seen later.