I'm starting to think the way to go isn't set stories in the sprint at all. There's a refined backlog in priority order. You grab one, do it, grab the next. At the end of the two week period, you can still have a retro to see how the team is doing, but don't worry about rollover.

Alternatively, don't think of Agile as a set instruction manual, but rather a group of suggestions. Have problem X? Solution Y worked for many teams, so try that.