making Registration End date relative
I feel this is very difficult to have it as part of Civi tar ball because there are many things we need to consider and come up with generalised solution. eg Algorithm to set start/end date when copying an event. How many org want this to change.
Instead i would just create an extension that would provide an option(pop-up) to set start date and end date (setting default value either today for start date and relative date to end date based on previous event dates or previous event start/end date) those field before a event is copied.