A Planescape or Spelljammer movie could be done almost completely in green screen, which could actually serve to greatly reduce costs.
And emotional impact. Moviemakers are finding that the more they use CGI backgrounds, the less the audience invests in the movie. It sounds weird, but apparently while we are willing to accept a little unreality in a fantastic foreground character, when you make the background world CGI, we notice and suspend our disbelief less. This may explain why, over the past decade or so, as expenditure on CGI effects has gone up, our enjoyment of the movies has generally dropped.