Hmmm I've been pondering something. But it's a pretty drastic change to the battle system.
You know how in the cartoon fish pokemon and stuff couldn't fight on land and land pokemon couldnt fight on water well I was considering this and belive it could still work in the games.
Obviously there are someexisting water pokemon who have legs and can fight on land. Kingler, Marril, Wooper, Wartortle, Poliwrath and all their evolutions can because they have legs. Wingull and Gyrados can beause they're part flying. Even the assorted seal pokemon and Shellder probably could. So there could be these different classes of water pokemon, those that can fight on land and those who cant.
So basically when going surfing you will only have water pokemon with you but you can still have a balanced team if they try to diversify the secodary elements of water dwelling pokemon. We have Lanturn who is part electric, Sharpedo - part dark, Dewgong - part ice, Tentacruel - part poison, Horsea is part dragon these days, right? Either way if they focus on creating many of the new pokemon as duel element water dwellers then we can further diversify all water battles. Not to mention that all the land water pokemon can fight in water too.
So if you get what I'm saying, I'm proposing a greater range of duel element pokemon, having pokemons abilities greatly effected by their environment and by doing so making players create alternate fighting teams making us diversify our lineups and learning the ins and outs of more pokemon. It doesn't just have to be water and land either. Some pokemon shouln't be able to fight in caves (large birds come to mind) otehrs could have problems with altitude and thickness of the surrounding vegetation.
This would add more depth to battles which is needed to negate that ever annoying tactic of little kids who just train the one big Pokemon.