I'd leave it as it is tbh. I know the ending didn't go down well with most people, but i liked the ambiguous nature of it and how it makes you wonder what really happened.
Besides, most, if not all of the storylines were wrapped up, so a movie would have to be standalone and i just don't think that they can squeeze all of the stuff that makes The Sopranos so awesome into a 2 hour film. Of course, we could get a "What Happened to the Russian in the WooRAB?!?!?" spin off, lol.