I have a suggestion for how the logic of scheduling and gannt charts should work.
I'd like to set estimated times for tasks at an early stage in my planning, then create the dependencies and see the tasks get lined up in serial or parallel on the gannt chart. Right now, the estimated time doesn't seem to get used for anything. This would call for a new parameter to specify how many working hours there are in a day, which might eventually develop into a per-resource holiday schedule etc.
A "likely end date" could be calculated for each task, equal to the likely start date plus the estimated time. The likely start date would be latest of the likely end dates of the preceding tasks, any assigned start date, and any assigned start date of its supertask/project.
As we know from bitter experience, due dates exist only in the fantasies of PMs, so the above should be the main scheduling model. You could then use the due date to paint things pretty colours. For intstance, if the likely end date is later than any assigned due date, then the bit between the two (i.e. the later part of the bar on the gannt chart) could be painted red. In the opposite case, there could be a faint green haze in the region from the likely end date to the due date, but it should not look like part of the bar.
A supertask (i.e. one with subtasks) doesn't need start or end dates. But if it has an end date, then the same colouring principle applies to the bar that represents the supertask, but there are no consequences for the subtasks. If it has a start date then it would act on the subtasks as explained above.
It's completely daft at the moment that if I define a subtask without a start or end date, the supertask disappears from the gannt chart, its start and end dates are wiped out and the subtask is scheduled to start in the year 5000.
I realise that some of this is within redmine itself rather than the better gannt chart extension, but perhaps it would be possible for both teams to work together on this.