Give the option of allowing/not allowing students to write tests after the posted due date without having to manually allow each student individually.
Ex. tests could be assigned either a "hard" due date that locks the test after the due date (i.e. what Populi does now), or a "soft" due date that keeps the test open past the due date but flags late completions (like Populi currently does with file-upload assignments).
The current "manual extension" feature should still be kept as is.