Latest Microsoft gaming rumor on the block is the surprising idea that the company is planning forward-compatibility for Xbox 360 games played on the third-generation Xbox. Where backward-compatibility is a console's ability to play titles from a previous generation of hardware (e.g. original Xbox games played on the Xbox 360, either through hardware or software emulation), forward-compatibility would be gameplay, graphics, control and other features of a title "unlocked" by virtue of the next-gen console's improved abilities.
That would mean Xbox 360 titles on the next-gen Xbox - let's call it the Xbox 720, though obviously that won't be the actual name - would have improved draw distance, higher frame rates and faster speed, together with taking advantage of new controller functionality and other hardware developments. You could even access new levels via DLC.
Microsoft are no-news policy on the Xbox 720 release date, but the latest educated-speculation tips it between 2010 and 2012. According to the sources behind this talk of forward-compatibility, the company is in the final "testing the waters" stage of deciding hardware specifications for the new console. At that stage it can explore the concept of the Xbox 720 mining new gameplay from existing titles with developers; there's of course no guarantee that the project will make it past the "under consideration" stage.