Currently, it seems that the only way to add a problem to a TO-DO list is to submit an incorrect solution to the problem ! It will be nice if there is a "TODO button" besides each problem. This way we will be able to save the interesting problems that we want to solve in the future without actually submitting an incorrect answer.
My TODO list is solved with all of the problems I haven't successfully solved yet, not problems I want to solve.
This is a cool feature request that would replace my notepad++ document listing the problems to work on.