In the 3rd generation, they added weather to the games. In the 4th generation, they added day/night to them and in the 5th generation they added seasons. What can they add in this generation? I can't really think of anything...
Hmmm... how 'bout strong winds? It could lower accuracy of special attacks, as well as speed of lightweight Pokemon.
Other things I'd like to see:
:: Low and High tide, which was present in Shoal Cave. Offers more backtracking at areas.
:: Also, if rain, sandstorm and hail naturally occurs, they should have places where intense sunlight naturally occurs. Only Hoenn has that. It would be great to have especially during Summer.
:: I can see Fog returning, too. There's fog in Unova's Route 15, but it didn't have a battle effect.