The Original Pokemon Anime Series was originally intended to finish in a half-year season with Ash becoming the Champion of the Indigo League following the original plot of the games Red/Blue/Yellow or in Japan as Red/Green.
"Pokemon The First Movie: Mewtwo Strikes Back" was originally created to be a sort of final ending to the series as it can be seen here it was different from the actual canon-version. The Trailer shown above is what was supposed to be the original ending to the Anime franchise.
Evidently it takes place in the future where Misty is an adult and a child that is supposedly Ash/Misty's daughter. Ash however isn't present probably following his destiny as a champion of the league in distant lands, leaving Misty, his Pikachu, and his daughter alone looking at the sky.
The rocky place in the trailer is around Cerulean Cave, which could've been the main setting of the movie, as opposed to New Island. The floating rocks Ash, Misty, Brock and Psyduck are running from could be going all over the place as a result of Mewtwo's psychic powers. The lab in the trailer could be the Cinnabar Island Pokemon Mansion, as opposed to the New Island Lab.
Remember, with Ash beating Gary and winning the Pokemon League, and them going on to face Mewtwo, they probably wanted it to be closer to the games.
i want ash to grow up a little and hook up with misty.
