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- mysite.com
Then the category1, category2, etc .mysite.com
When a user signed up they would be under mysite.com/user
And their blog/site would be category.mysite.com/user/site
Am I making sense? Is this something that seems realistic using what's available on wpmudev?