If they continue to make games in the manner they have for the past number of years, it's likely that they'll keep to Grass/Water/Fire. But if they were to finally decide that they wanted to do things a little different for one of their games(I.E. a different perspective game like Pokemon: Rocket Version, starting off as a Team Rocket Grunt.), they'd be more inclined to change the starting types to something suitable.
Emerald wouldn't likely have any changes made to it by way of starting Pokemon, though perhaps they'll do something like they did with Yellow and have you start with a different Pokemon entirely, not letting you choose.
Future versions of the game could and/or should have changes in the starters. If they were to make a game where you were to start in the Orange League(Though you'd be able to travel elsewhere afterwards), the logical choices for starters would be water types that can already Surf, unless they expect you to travel around on a boat to each island, which could take away from a player's interest.