I've actually thought many times about a combination sequel, but they just keep adding to the story. I figure I'll wait for VII, which they are saying will be the finale. (At least for the current storyline, I'm guessing.)
Not that I'm expecting much from the next one, but it's always good to have more footage to work with. :)
Out of curiosity...[SPOILERS FOR VI]...
If you want to only use III-VI, what would you be doing with Hoffman's open-ended fate from VI's climax?