Cool stuff. Although that's the first-est tiniest step to a design doc.
Three things to do in parallel:
1 - start coding something SIMPLE to get yourself familiar with twine. Keep it super simple: one stat, two locations, a day loop, one or two activities (a grind task and one reward scene you get after grinding 3 times)
EXPECT TO THROW 90% OF THIS AWAY - it will be nasty and bad, and you don't want to keep working around your early mistakes forever. But you will learn enough to do it better on second pass, the real deal.
2 - deeply think about the scenes you want. brainstorm and write down lots and lots of ideas. keep each very short (1-3 sentences)
This is probably the fun part. Once you have plenty of scene ideas (20+) then you can pick some and polish them up with more detail. start to see patterns how the scenes could lead to one another, so you can plant clues/joining logic in your story.
Finally, when you are ready to make the actual game, use a few of these as your starting points for dialog and action.
You can combine this step with also writing more and more background info about the specific characters in the game - MC of course, but also everyone else who is part of the plot. Look up the many many articles about character/plot developer techniques. What the like, hate. Their family and friends. memories from teen years. ambitions. style, music, habits. catch phrases. speaking style.
3 - deeply think about the game system. What exactly would a player need to do to progress?
typically you can gate progress behind three things:
- in game character knowledge (the bartender said I should talk to joe in the corner. a new navigation option appears)
- items (obtain item X to now be able to open the door to the secret room. "obtain" might mean buying something with some stat you have accumulated)
- stats: can't progress to the sauna room at the gym until your fitness is level 5
the "best" system is one in which there is some player effort but not stupid grind.
"sleep->gym->shower->sleep" 20 times to get to level 2 strength is STUPID.
But gaining points from story choices (and some minor grind) that somehow all fits together with the narrative and makes progression feel natural - that is CHEF'S KISS AAA game design.
FINALLY: get some art sorted out. But if you are using stolen realporn etc, this is like 10% of the effort, so don't prioritize it. Exception to this: maybe get some inspiration pictures for each character, so you know what they look like.