Advise #1: Why not add a button of "Try the output of this input" ? You can give an input, and the checker shows the output or "invalid input" . Sometimes to check if the output of our own code is correct, we'll have to calculate it by ourselves. Adding such a button is useful then. Of course, it should be forbidden in contests and virtual contests.
Advise #2: Sometimes the user doesn't want to share their code. We can add a function like Vjudge. The user can choose if they want to share their code or not.
Sorry for my poor English and poor rating :P