I feel like reddit tried doing just this. You see this quite clearly when you visit niche subreddits. The problem is that the large subreddits have a larger imp

I feel like reddit tried doing just this. You see this quite clearly when you visit niche subreddits. The problem is that the large subreddits have a larger impact on the direction the site takes. They make demands of the admins and they spread their culture to adjacent subreddits. The barrier between two subreddits is likely lower than two websites.