>>42215435at first it was just HC's leak with the extra key items lightly describing a not to dissimilar plot (there's a "Fuji Letter" key item too, so maybe all the Pokémon Mansion bit was less vague about Mewtwo's creation and birth) and the encounter table featuring Mewtwo as a post-game encounter and such, but then I realized the anime ends with the gang visiting a remote island to fight Mewtwo and I guess Mew was there too. Doesn't the movie even starts with a Dr. Fuji losing control of Mewtwo who destroys a laboratory while escaping? For some reason, the idea of going sailing to a faraway island after the league was so prevalent FRLG implemented it as post-game content, one of the TCG's first promo sets was about remote islands, and so is every plotline in the anime between Kanto and Johto. Considering the game itself was meant to finish with a post-game island, all of that might've been based off or at least inspired in an earlier iteration of the game's plot.
This is mostly speculation though, Mewtwo apparently was coded as a wild encounter on the uppermost floor of the scrapped Silph Tower on the leak, according to the encounter data text files included. In a way, you do visit Cinnibar, a faraway island, when going through Mewtwo's plot, also there's an unused "hermit" overworld character near the end of the sprite sheet so maybe the trip to the island was meant to be something else entirely.
