Worldbuilding has a sandbox. I’m quite familiar with it.
Because of the limitations of SE, the sandbox is implemented as a meta question (“this is the sandbox”), and question drafts are answers. An answer contains the question draft, proposed tags, and any meta-commentary from the author (what you’re having trouble framing, questions for reviewers, etc). People then respond in comments. Iteration happens. Eventually the question gets either posted or abandoned, and deleted from the sandbox either way.
In a Codidact sandbox the post is a “question”, but not a question to be answered in the sandbox. It’s more of a request for critique, really, same as someone in Photography asking for help improving a photo or someone in Graphic Design asking for help with a logo or someone on Writing asking for critique of a poem. These aren’t really questions; they’re requests for help or input. I would implement that as a “discussion” post type.
Which is exactly why a “one post type per category” approach doesn’t work. Communities should have the option to have those canonical wiki posts – which are not Q&A posts but another type – right there with their Q&A if they want. And meta could host Q&A and discussions and announcements. And they might have other wiki posts that don’t belong with their Q&A, and blog posts that do or don’t.
A category defines the content that goes there. Which specific post types implement that is orthogonal.