In my opinion, Bagan would have never become as popular and unique as he is if he had been used in a film. In the Heisei era, he would have simply been a fairly typical, bulky, beam spamming, and forgettable monster, and if he was used in a film today, it would be almost impossible to make him impressive enough for the fans. As it is, Bagan with his video game appearance and countless fan fics doesn't really need a movie. Besides, older monsters and fresh ideas would be better.
Although I wouldn't protest if he appeared in a movie.
As for video games and anime, Bagan would work pretty well as the final boss/ultimate villian.
If you have nothing to lose, than you have nothing to fight for.