The existing communities here on Beehaw have broad topics anyway, and cross-posting isn’t allowed, so you can just choose which one is the most appropriate for your post.

If you think none of the communities here apply, maybe you can try posting to communities on other instances? (Someone correct me if I’m wrong here and it’s not possible: I haven’t actually tried this.)

Please consider that Lemmy is fairly niche already. Unless the topic of the community you want to create is popular enough to gain interest, it’s more likely to end up dead in months.