According to Kotaku sources, the next Xbox will be upgraded to Blu-Ray (finally) but it seems to be for reasons other than extra disk space. It looks like Microsoft is going to try using some large anti-piracy/anti-used game system that may take up a bit of space on each disk.
Microsoft also plans on releasing a new version of their Kinect that has an on-board processor. This should help the Kinect detect user motions more effectively.
You can read more into it all here.