For example at another domain like
stack.com: For smaller companies/ local regions: The ability to start your own stack project: Something like
As an example would be the Code Software, or a stack page for the Fandom Wikipedia, basically sites that aren't worth a proposal of a large community, but still need question & answer sites. Of course there will be a free plan that will be full of ads and such.
Another thing that you'll achieve with this is popularity.
The only big con I can think of is useless communities, but here I can think of something like forums activity. A new started forum has the ability for some time to evolve in popularity, and if it reaches a post count less than one per day for example, it'll be closed and archived, freeing the subdomain for other users. This way you can also see which community is actually large enough for attention, and which is not.
The creator of the stack page will only be given some start reputation and some high access, the rest goes democratic like in any stack overflow page.
P.S. Not sure where to post this, and what tags to include, but since area51 is new site proposals, it makes sense to me to do it here.