Have you tried making bldgs harder, but stay down 30 minutes instead of what they are now. It seems to me folks would tend to mass for an attack to kill things rather than piecemeal it, and give time for more planning, missions , etc.
Also, it would be nice if bombs disable flight ops for awhile but didnt kill the CV, and torps were required to actually sink it. That would give torpedo planes a real use, rather than 1 flight of buffs, or a series of kamikaze p47s wasting it.