4. Rural Locales
Almost everywhere you go in a Pokemon game, there's one trainer or another who explored the area before you, just waiting for you to walk by and battle. This creates the illusion that the Pokemon world is largely explored, and therefore takes away from the mystery of uncovering the secrets of each Pokemon region.
What's more, Unova, the latest Pokemon region, built its visual foundation on gigantic cities and bridges as Pokemon's American equivalent. X and Y appears to follow suit, being set in the France-based Kalos region. While it remains to be seen just how urban Kalos is, Gamefreak's X and Y trailers highlight its cities first and foremost. Furthermore, every Pokemon game sends players from city to city to gain badges. I don't know about you, but I feel the constant urban reliance is getting stale as the sixth generation nears.
With the addition of more rural, trainer-less locations in future games, Pokemon would retain a greater sense of adventure. After all, each game tasks the player with completing their Pokedex, why not also task the player with exploring unexplored frontiers for Pokemon instead of trainer-filled caves and populated cities?