It would be very helpful if Revit supported element reuse on large complex existing projects. Currently, there is no built-in way to track the removal and replacement of doors, equipment, etc. A manual effort is to create a set of custom parameters, but it is prone to error compared to Autodesk developing a built-in workflow, similar to what I suggested below.
Since element resue is related to phasing, I think there is a relatively simple way to implement this. Revit has two parameters used to track the phase an element is created and the phase it is demolished, with the latter being optional.
If an element is ever set to be demolished, it becomes available for reuse in future phases. When in a command to place elements, there could be an option to place available reuse elements... similar to how unplaced Revit room/space elements can be placed in lieu of creating a new element of that kind.
For consistency and scalability, the reuse item placed in a future phase could then also be demolished again, and reused in yet another future phase.
I am sure there are things I have not thought of, and elements within systems might have challenges. But I am sure Autodesk could come up with a thoughtful solution for this problem.