Using sub domains for main categories with sub directories for users?

This is solely theoretical at this point, but here’s what I mean: First, the type of network would use sub directories. Let’s say I wanted to have multiple main categories for users to choose from when (or after, either way) they signed up. These main categories would have sub domain pages that would initially be set up through cpanel.

So we’d have the main site-

Then the category1, category2, etc

When a user signed up they would be under

And their blog/site would be

Am I making sense? Is this something that seems realistic using what’s available on wpmudev?