Site Categorization?

I have no idea if this is possible, but here is what I'm trying to achieve.

Say I have a website that talks about cars.

The main site would be running the "network" theme and feed content from each subsite -- Great!

I want to achieve one of the following:

Option A:
Assumptions: Sites Pre-Created: "Hyundai" "Ford" "BMW"
1) Users can register and get to "pick" a subsite to be an author on when they register. Users don't create new blogs at all. OR maybe they could create a category if none existed?

Option B:
Assumptions: No sites exist
1) Users can register and create a site of their name/choosing (no problem). How can I both categorize, and display, sites of a similar topic?

Example: I have 10 users. 3 users create a site to talk about BMW's, 3 Create a site about Ford's, and 6 Create a site to talk about Hyundai's.

Network will show me content from each of their pages. Is there any way to specify SOMEWHERE a "topic" for each site, and then be able to build an automatic listing of sites broken down by topic?

The end goal is to have users be able to register themselves, and make sites specific to a topic, and when peopel visit the main site using the "network theme", be able to easily find the blogs pertaining to "bmws" etc.

I haven't seen anyway to categorize blogs yet though.