The purpose of the To-Do List app is to not only help the user to track their productivity effectively, but also help them to identify the habits, external influences, and possibly even health choices that affect their capacity to achieve their goals.
Elements of To-Do List (working title) app in order of completion:
Allow user to create daily, weekly, monthly, or even yearly minimums in terms of productivity
Record whether or not goals were completed within the time frame alloted by the user
Use data to populate a graph to show your consistency of task completion over time
Create an option to record daily moods, and possibly an option to explain the context of each mood in short blurbs
Create a mechanism that reminds you of goals periodically to make sure you're on-task, by e-mail or text, or whatever would be most effective
Ask user if they would like to challenge themselves to something new if they are consistently attaining their goals over a certain period of time