As someone said already, Yesterday's Enterprise (with the 1701-A) would be my first choice for a script they adapted into the first movie. Another interesting idea was one that I think I read somewhere was the original concept for "Too Short A Season" in an early draft (or was that just a rumour?) - that instead of Admiral Mark Jameson, we would have had Admiral Kirk, very old at this point, travelling with the Enterprise-D to negotiate the release of hostages on a planet he once visited. The plot would have needed to be expanded, but I think the idea of Kirk taking command of the mission like Jameson did, with the tensions with Picard, would have been an interesting take... and if Kirk was having to deal with consequences from some actions he took as captain of the original Enterprise ... it could have led to some nice introspective moments with Picard. The whole idea of the Admiral taking an alien drug to make himself young enough to go on missions again would have been an interesting theme as well, considering how Kirk seemed to regret aging in some of the original movies (e.g. The Wrath of Khan). Jameson certainly seemed as though he was written with the intention of being a Kirk-like character. Either way, if they had made Yesterday's Enterprise or Too Short a Season into a movie with original series characters, they still could have called the movie "Generations" :-)
(... and just to throw it in, if they were making a novel into a movie, I think Peter David's Vendetta would have been a way better Borg movie than First Contact, too!)