Since we're all contributing, I've got a little pet idea for how the overworld of Pokemon Black and White should play out. Specifically, I think Pokemon themselves should be more prominent on the overworld - you'd be able to see Pokemon in the wild before you battle them, HM's would be more useful, the follow feature would return and so on. As some example pictures, here's some concept shots I made a while back for a Pokemon RBY fangame I was going to make:
The first picture best shows what I'm talking about. Instead of random battles, you'd actually see Pokemon on the field, some of which would actively pursue you and some of which would be shyer, making them harder to catch. They would primarily stay in the grass as before, but some Pokemon would make a stand and move onto flat land, and some Pokemon would hide in trees, swooping out when appropriate. Pokemon would of course be able to follow you when you've caught them.
Second picture shows how I think surfing should work. Firstly, you should be able to see the Pokemon you're surfing on, not just some blue thing. Secondly I thought it would be a nice touch if you could sort of "see" underwater Pokemon moving around. Mostly Tentacools and such, but deeper down, you'd also see schools of Goldeen and Remoraid, as well as rarer water Pokemon, which you wouldn't be able to reach unless you fished for them.
In the third picture you can again see the whole "Pokemon on the overworld" aspect. If you were to touch one of those Digletts, or the Digletts were to touch you, you'd fight them ala Paper Mario. Obviously wild Pokemon would be fast and move tactically though, so it still wouldn't be easy to walk through grass or a cave without any wild encounters. Also to note in this picture is the fact that two of the Digletts are facing each other, one being a shiny. I thought it'd be good if wild Pokemon "interacted" in a basic way, and I also thought it would be awesome to see shiny Pokemon in the wild before battling them, although they'd be crafty and try running away from the trainer.
Finally, a more experimental idea. In the shot you can clearly see Red and Erika riding on a Parasect and a Venusaur. The basic idea here was "what if you could ride your Pokemon?". We already have the bike, but I imagined it would be appropriate if you could also actually "ride" certain Pokemon (mostly fully evolved ones, fast ones and ones that are built for it i.e. Ponyta). There could even be races too. Not just that, there could be other uses for Pokemon on the field too, more than just breaking rocks and climbing waterfalls. What if, when in tall grass, your follower Pokemon could actually go off on it's own and "hunt" rarer Pokemon and corner them for you? Or what if there were Pokeathlon-esque events that took place on the overworld?
And the thing is, these things wouldn't even be hard to do. It's just taking existing Pokemon overworld sprites (and making new ones for the new Pokemon), and giving them basic AI sets. Certain franchises allowed us to "see" random encounter RPG enemies as early as Adventure Of Link and Earthbound. Putting a handful of Pokemon shadows underwater is not much harder than implementing random battles, allowing us to see shinies on the field would only be a good thing, partner Pokemon were already in HGSS and being able to allow us to do things like "riding" Pokemon would be no harder than implementing the bicycle. But these little things would help make the Pokemon would feel more real and less artificial. That and finally granting us diagonal movement and proper animation in battles.
One other thing; I'd definately like to see the PC implemented in a Pokemon Ranch style, with an option to see your box 'Mon frolicking with one another and interacting.