When planning a visual novel, one questions that rears its head is actually how much art do you need? What characters are important enough to get portraits? What poses should they have? How many pieces of background are needed, and for which areas? What about CGs? How many and for what scenes?
The easiest way to go would be to finish writing the entire story first and go from there, using it as a reference. The only problem is that doing is that way would drastically delay the game’s completion compared to working on everything at the same time.
The other big concern is budget. Each piece of artwork costs money and every game has a budget. In the case of Aurora’s Nightmare, a fairly small one. So I can’t necessarily get artwork for everything I want. So I need to figure out exactly how many I can afford and then prioritize. So, while I may not have every piece of art I’d like, I at least get all the most important ones.