A task can already be given a priority but the Gant diagram only moves a task after another if it is given a predecessor. It would be better if one only specified predecessors for those tasks that really require another task to be completed before they can be started. 
This would allow one to see easily how assigning a task to a different developer could affect the timeline. At the moment one is forced to add an artificial predecessor if two tasks are assigned to the same developer. This means that if the second task is assigned to a different developer the tasks won't now be done in parallel but still sequentially.