I think one thing that might be helpful is to think of a system not just as mechanics but also a theme, like space horror, black metal fantasy, or exploring dark and mysterious woods. So rather than thinking about the mechanics, make up a setting to put your entry in. I think your question is probably one a lot of people have, so maybe the jam is a way to break the assumption that a system needs to be fully fleshed out or that you need a strong understanding of system mechanics to make ttrpg materials. This could also be a way for you to take the first step towards designing your own system without having to commit to writing a whole ass book. Doing research for this project I’ve found some people who do just that, they’ll make references to a game book before it’s come out but eventually they will publish it. I think the barrier to entry for writing a module is much lower than for designing a fully fledged system.