That's a little different...

Reading your post a few times, I don't really see the difference. Your last sentence perfectly summarizes small subreddits and cross-posts.

Reddit doesn't prevent more people from joining your sub. If you want a sub of 10K users, and 10M people join, too bad. Secondly, it doesn't filter users based on location. If I wanted to talk to the nearest 100 people, too bad. I have to make a new sub, find those people, and convince them to join.

So really, all of the two things that I suggested are things that reddit doesn't do.