For sites with large sitemaps and continually updating content (like news or blog sites), it would be helpful to have content/collection based sub-sitemaps and have a parent sitemap index. Most e-comm and news sites use this technique to help Google crawl relevant content updates more speedily and re-submit sitemaps when content is updated. It would make sense to have the option to either have a site wide sitemap option or an option to have multiple sitemaps including static or dynamic CMS content based on collections.