How Mythical Pokemon Often Hint at The Next Generation of Games

gamerant.com:

With Pokemon becoming a worldwide phenomenon where every major release sells millions of copies, the franchise is one of the biggest on the market. It is widely assumed that Generation 9 Pokemon games are in the works despite no official announcement, simply because it's been the trend for Nintendo and Game Freak to release a new mainline Pokemon game every three years on averag. However, this wasn't always the case, and there was a time when there was no guarantee of another Pokemon game. Yet there was a pattern that indicated which ideas The Pokemon Company wanted to explore in the future.

While Mew and Celebi didn't necessarily hint at Gen 2 and Gen 3 Pokemon games, it's very likely that the Mythical Pokemon from Gen 3 onward did come with clues about the following Generation. It remains a mystery whether these Mythical Pokemon were designed specifically to hint at future games or if they were conceived at a time when Game Freak was already working on the next set of titles, making their designs inherently more in-line with the themes of following games. Yet, this became a pattern that can often accurately predict what the setting for new Pokemon games could be.

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Mythical Pokemon are basically like Legendary Pokemon, but usually released to the public through events, Pokemon movie tie-ins, and the like. ForPokemon Ruby, Sapphire, and Emerald, the Mythical Pokemon were Jirachi and Deoxys. The overall themes of Pokemon Diamond and Pearl were creation, space, and time — embodied by Arceus, Palkia, and Dialga, respectively.

Space is something both Jirachi and Deoxys incarnate to an extent, both in terms of the vastness of space and ideas about heavenly places.