I think that a fair bit of the functionality in Assignment needs to be pulled out and made more generic for all activities to have access to. Blind marking and team grading come to mind, and I know i have put in tickets requesting this from Daymon.
I also think that all assignments should have the option for due dates, cut off dates, and extensions. Both quiz and assignment have these implemented. Seems like time to complete would need to be added to each of these dates as each could be different, except extension as that is based off the cut off or due date.