Scheduled vs. Actual issue time
Redmine is are perfect instrument for developers but when I try to use it in our company I meet that our project managers needs additional functions. Most important of them - ability to view real project progress compared to planned one. Here should be two independent date pairs to implement this - start/due dates for planned issue bounds and started/ended dates for actual. Started and ended dates can be set automaticly when issue status changes New->AnythingElse and NotClosedStatus->ClosedStatus respectively.
Additionally second bar (optionally) should be added to Gantt chart. This may look like here: