I think it would be useful to add a feature which allows customization of a practice test with user selectable topics that could be used to prepare for important tests. In my case, I would like to input the number of total questions, a time limit, a list of topics to select from and, perhaps, a selection of difficulty level. This site would randomize questions from the selected topics generating a different test every time. Such an option would be extremely helpful because time would be focused on practice for your specific goal.
Thank you for your consideration,
PS: my parents would consider donating for such a feature development. They say "a feature similar to Go Fund Me internal to Khan Academy would allocate development costs on features wanted by the KA community".