In my opinion, there will come a time that Gamefreak and Nintendo would be out of ideas for a new pokemon and new region.. I think that time, they will think of something that would sell until they get new ideas for new regions and pokemons. That time, I think they will try to make a pokemon game with title of something like "Pokemon Generations".
Containing all 5 regions (Kanto, Johto, Hoenn, Sinnoh and Unova) with extra regions (Oblivia and Orange Islands and some random island or city or half a region) and with 1 ultimate legendary and a feature where you can catch all the possible legendaries like (3 birds, 3 weather, 2 towers, 3 golems, 3 spirits, 3 creations, arceus, zek and resh and more..). And also in my opinion, it would also contain new storyline about the ultimate legendary and new trainer system feature (where you can choose from 4 different types of trainer - 2 males and 2 females each with different story backgrounds), pokemon musicals, pokeathlon, contests, etc..
it would also contain of course a graphic enhancement (pokemon 3d), new battle system and also in 3d, new villians and a new evil group called "Team [insert name here]" that plans to conquer the world by befriending the ultimate legendary that came from another world but will be abolished because the ultimate legendary is planning to destroy the world itself. Then the 4 trainers will combine their strengths along with the gym leaders, ordinary trainers, and the evil team that is destroyed by the ultimate legendary. Arceus, Mewtwo and all legendaries will appear in a movie cut-scene and all will battle that ultimate legendary then BOOM... *epic*
and it will be sold in 3DS and the 3DS sales will go BOOM! because of that game... Box art will be cool, and its color will depend on the color of the ultimate legendary and it will be the displayed mascot in the front of the box. It is possible also that they will make another pokewalker thing or accessory into that game.
Sorry.. seems like my opinion just turned into a speculation, anyways, if they made a pokemon game like that then, it would sell million of dollars or [convert currency]. I for one will buy that game whatever will be the price... And I think that will be the downfall of Sony err i mean PSP...