One of the things I ran into while trying to make a 'News sitemap' sub-module using the new context system was that there will be a conflict as several different modules try to provide different sitemap types that cannot be combined. We'll need an elegant way to handle a global 'type' context (for XML sitemap types: normal, news, video, mobile, etc.).
I'm also running into the problem that we need to allow the XML delivery callback to take an argument of contexts as well if we wanted to add a new menu item like 'sitemap-news.xml' that would automatically have the 'news' type context be TRUE.