I would also love to see "Time to Complete" as relavive date in Moodle. I imagine that this should really be part of the conditional activities, so for all the activities - not implemented separately for each.
For example, in section "Restrict access", for allowing access from/to - you could set up dates relative to your enrolment time, not just absolute dates. You could say that activity X is available to students X days after they enrol.
"Expect completed on" could also be set mean "students are expected to finish this activity within 10 days", instead of just a date.
The extension of this would be to move options in assignment module for "allow submissions from", "due date", "cut-off" date into generic module options (and then have each module to support those or not). This way, a common notifications framework could be easily built in Moodle (e.g. send email 1 day before cut-off date for any activity).