It is nice to have grade indicators such as missing or incomplete on assignment, but I'd like to propose that the gradebook administrator be able to define this list of indicators. Furthermore, the gradebook administrator be able to define how these special grades interact with the grade calculation. Some examples would be:
Exempt: handy for assignments that should truly be exempt.
[Value]: where the entered value is the score of the assignment (I would put 0 here for missing and incomplete).
[times a multiplier]: so that a school's late work policy can reduce the grade by a certain percentage (e.g. if a school's late work policy is 20% off then you'd put an "x.8" here).
Like the grade visibility settings, it would be nice if the gradebook administrator could either make these settings school-wide or let classroom teachers make their own decisions.