What if, between the events of Star Trek V and VI, what if a Romulan vessel attacked the Enterprise-A, completely destroying its saucer section? (But luckily Scotty managed to beam Kirk and crew to the secondary hull just before the saucer exploded) What will happen next?
But what would happen if Starfleet changed the design specs in the meantime and the new saucer doesn't fit on the old stump?