A document was released a few weeks ago that purports to be the Microsoft plan for their next XBox.(direct download)
CVG did an analysis of the document and claims that it is real. Although the document is from 2010, it is likely that, while some of the specific details may have changed, reading through this document should give us a good idea of what to expect in the next XBox when it comes out (likely) for Christmas 2013.
The full analysis can be found in this article.
The most interesting things to be included in the next XBox: