I would like to see the ability to automatically create an XML sitemap “on-the-fly” built into the CP core.
As with the ability to edit the page title and meta description (see https://petitions.classicpress.net/posts/175/allow-editing-of-page-title-and-meta-description-seo) , a well-structured and up to date XML sitemap is an essential part of SEO, helping Google (and others) to crawl your site.
This feature is only made possible at present through the installation of 3rd party plugins but in my view should be a core feature of any business-focused CMS.
If this feature were to be implemented, there could be global enable/disable setting and/or filter to be used as and when required (e.g. when installing a SEO plugin).