With anything but a Philip K. Dick novel I'd say okay, but not with him. Dick was one of the very few SF writers,(or really, ANY writers) who produced a body of work that could be looked at, really, as if it was a just a treatise with one, (or maybe two, but they're still highly related,) major theme. Dick, really, was batshit nuts (but in a good, well, at least a controlled, way), and his whole life was bound up with (and largely ****** up too, from what I've read) with this sort of constant obsession he had as to what constituted reality. Each of his books look at this question from a different angle, like a jeweler "romanicing the stone" that is, looking at it in a variety of angles and seeing constantly varying changes all the time. So no, you can't make a real sequel to any of his stories because they're all really sequels to each other anyway, if that makes any sense.
You COULD make one, I guess, and explore the same aspects of the story, and if the acting, fx, production, etc. was any good it could be good, even better than the original, but it wouldn't ,IMO, be a PKD movie, if that would make any difference to anyone.
Now VALIS? For the life of me I can't see why no one has made VALIS into a movie. It would be the PERFECT vehicle for Terry Gilliam.
