Orphaned structures
A small change I would like to see in sins is for structures (and possibly other objects) to slowly lose HP after their "parent" has been destroyed. It would work something like strike craft that have their host destroyed in which about every second or two they loose a percent of their HP.
This could be explained away as supply, repairs, and maintenance have slowed or stopped and as a result the machines slowly breakdown.
An example that has happened to me - I have taken a planet but do not want to waste the time clearing the remaining buildings out (as I am trying to chase down a fleet), I would like these buildings in my new grav well to slowly die.
Another scenario could be to let the above happen but if these enemy structures exist within the enemy's starbase area of effect this building could be said to have taken control of these other structure and is now the "parent" and the slow decay of the structure doesn’t occur.
Yet another option could be letting the construction frigates have a deconstruction ability to dismantle the buildings or take them over.