I realize that this is an unusual suggestion for a tracker but I believe it does go with the mood tracker well, can be implemented very easily, does not require any input by the user and can be used to predict or even change a person's mood.
My idea is a music tracker.
Most music programs have APIs to export played songs. Some have APIs for input as well. Combining the data from the mood tracker and the music tracker, MedHelp could determine what type of music, artist etc. puts the user in a better mood. At the very least it could be graphed, at a higher level it could be used to have the music player play songs which would increase the user's mood.