I think the best alternative here is to use a theme component, Discourse Category Sidebars could also work. You can set the sidebar to the parent category, and then create a topic with a list of links to the subcategories. You could even include the “Add a new idea” and the “Filter by category” parts.
It would require some manual work to set it up though. And you’d have to fix it every time a subcategory changes.
Other than that, this could be done with custom development work. If your site is hosted by us, you can send us a private message, and we can talk about it. We’ve done very similar work to this for other customers.