Offense/Stronghold in Co-Op is fun, but having the option to start defending against waves of enemy numbers would add another take on Co-Op gameplay.
Just attempt to test the Defend/Entrenchment Co-Op by replicating what DOI has done and see what happens. Now that we have fire support, it'll be very useful defending in Co-Op; it'll give a chance for new players who start playing Commander a chance to play around with fire support.