It would be good to have external fuel tanks (such as on the Space Shuttle) whereby the fuel is in a predefined order, maybe set in staging, rather than setting the tanks to fill and drain which just decreases the rate at which the main tank is used, instead of deactivating it until it is needed.
That doesn’t allow a fuel usage order, merely a change in the size of tanks? An idea that would be useful is to have interstages, and fuel interstages, where the fuel is used first in the fuel tank that will be jettisoned, then it can be interstaged as per usual. @Kell