I plan on writing a static site generator soon myself. And I briefly wondered the same thing and realized that I could very easily answer that question: ruthlessly build whatever I personally need and not what others need. I don't
usually follow that line of thinking, but the static site generator space is so full of so many great tools that I don't think there's really anything for me to contribute to it.
Instead, I think the killer feature of my static site generator is going to be exactly what makes it difficult for others to use: its tight coupling with my specific use case. I'll be able to hard-code configuration knobs. Avoid dealing with edge cases. Bake strange non-generic and single-blog-post-specific logic right into the tool.
(The reason why I want to build my own is because I've tried using someone else's. And I just keep getting bitten: someone else's tool gets so big and moves so rapidly, that by the time I get around to updating my site, something about the new version of the tool has broken the way my web site works. And then I usually have to spend hours figuring out how to fix it, which is hugely deflating because I had just worked up the energy to write a new post. My naive belief is that if I have my own tool that evolves at precisely the pace I want it to, I won't run into that specific problem. I'll run into other problems of course, but I'm thinking that I'll prefer those problems to the ones I currently face.)