This might go well with a previous suggestion to allow definition of "rules" or macros to have changing of some properties also change other properties automatically. I think this kind of power in MLO would be consistent with other features (ie, not feature creep) and would have a lot of broad uses.
I'm thinking of something like a rule/filter editor where you have the choice of:
"When" X (new task created, project attribute set, task completed -- this list could start basic and be expanded)
"Then" Y (change attribute A to value B)
(Personally I like Eudora's method of doing this, where each rule has an option to "skip rest" so that you don't apply later rules).
A lot of the code for advanced filtering could probably be tweaked to do this, since there is already support for mapping various task properties (e.g., Goal vs Completed) to the individual values they support (Week, Month, Day versus Yes/No).
In summary, I think a rule system would be an elegant/expandable model that could also cover default values.