Project files working sets.
IOW, being able to save the set of working files in a project (with a name, preferably) while I temporarily go work on another, higher priority bug. This would allow me to come back to the same context in this project when I'm finished with the "higher priority" bug.
This would also work well when your work flow is to have a separate repository clone for each bug, or even a different branch in one repository. You could suspend working on a bug while you change repositories or branches, work on that bug, then switch back and have your working files exactly as you left them.
BTW, SE works seems to work fine if you change a soft link underneath it, which is a good and useful thing. For example, I can change to a different repository clone (ie a git or mercurial repository) by just changing a fixed soft link, then switch back by just changing the soft link to point to the previous repository. This feature would be much more powerful if I could save the working set of files and their current append points and then restore them, as described above.