I had devised a concept for the end of the film... Some alien form has designed a way to use the power of a black hole's gravity to form a spherical shroud around the black hole. If you have a dense enough material, gravity cannot penetrate it. There are two openings in the shroud that they would use to pull ships in. The saucer of the Enterprise (which was detachable) ends up in the shroud. They meet the aliens and had a dramatic finale. These ... images are of the Enterprise saucer in the shroud - Ralph McQuarrie