I once tried out a novel-writing package, to see what it was like, but I couldn't manage to understand it at all. I think one essential thing is to remember that the people who are using it will not know things that seem obvious to computer-savvy people like the designers, so make it really idiot proof and make sure that it is logically organized as possible, with a tutorial to walk you through it and so on.
I can see it having a market, perhaps with less confident or beginning writers.
The trouble is that so much of writing is intuitive and a computer just can't help with the important problems. To take a small example, Word doesn't pick up on mistakes of context - like if you type 'He peaked through the hole' instead of 'He peeked through the hole', that won't be registered as a mistake. I know you're not planning to make a word processing programme, but it's an example of the limitations of computers when it comes to writing. If a computer can't pick up a context problem, will it pick up a continuity problem, and if not, will it be able to contribute much to the organisation of the novel? Sorry, rambling a bit here, but hope you see what I mean.

Thanks Jess, had a look at Scrivener and downloaded the free trial. Am impressed.