I've been mostly playing with illustrious lately and had a version of this up for a bit but I think I got it to the point where I'm happy to share it. One of the main things for me is I have it review what you want, and then "fill in the blanks", it will automatically add details that fit with the scene. If that isn't what you want, you can tell it not to do that.
One thing I've been noticing with GPT lately is that it will often get stuck on previous details on the chat especially if you iterate on an idea a few times, which is something I like to do and I'm happy with how this one responds. Just make sure to make it clear that you're doing something completely different if the next idea you have is a whole separate idea.
Please feel free to leave comments on what you like and don't like and I can continue to iterate on this GPT for a better user experience.
One of the things I've learned is that it's better to put as much as you can in the instruction prompt; I used to put the bulk of the instruction in a separate document, and the instruction prompt was a brief overview directing it to review the documentation. I've found that it retains the instructions better if it's all in the instruction prompt, but as of writing, that field is limited to 8,000 characters. My original instruction prompt was 11,000 characters and I had to trim it down in order to fit it in.
Below are images I generated using this GPT:
