An Inter-region game is a Pokémon game that includes multiple regions. As international and interstate refer to multiple state inter-region can be used when referring to the different Pokémon regions.
Pokémon Heart Gold and Soul Silver (remakes included) and Pokémon Crystal are inter-region games because the trainer can travel to both Johto and Kanto. Pokémon Heart Gold and Soul Silver (remakes included) and Pokémon Crystal are the only true inter-region Pokémon games.
Since Pokémon Ruby and Sapphire many Pokémon fans have desired another game like this. Now certain people want a game including Kanto, Johto, Hoenn, Sinnoh, and Unova. Pokémon Fire Red and Leaf Green also create the desire for the Sevii Islands to be included. Several fans have created and submitted their own ideas for this. Following the Pokémon Heart Gold and Soul Silver and Pokémon Crystal example, the fundamental Level 100 would probably have to be broken and new story-lines would have to be created in the non-starting region. Evolution levels could be changed too, but it's all up to the fans, programmers, and story writers really.
On the concept of it being a platformer, one fan came up with a game idea starting in Sinnoh (near the Platinum story-line) to allow the import of a single Pokémon Diamond, Pokémon Pearl, or Pokémon Platinum game data per save file. This process would preferably not delete the DS save. The fan made a website... (Need to find website)