Totally agree - having a way to add relative adjustments in both volume and duration for the selected notes would be great. It would preserve dynamics when changing expression markings.
It could take the form of a dialogue box with a percentage adjustment e.g Vol: +10%, Duration = -5%. Those values would be stored against each individual note, and could also be individually adjusted as they currently can be, but using percentages instead of absolute values.
Existing scores could be converted automatically between the old absolute system to the new one when loaded.