I'd quite like to see something that extended the life of the main game as well, either through additional storyline scenarios that drew you away from the "main quest" as such, or additional obstacles thrown in your path. Despite their open world policy, Pokemon games have always felt very streamlined to me: you go here, then you go there, and so on. If you deviate, you get a line of random NPCs blocking your way, none-too-gently pushing you back onto the path the game wants you to follow, taking your hand quite firmly. I'm not saying this is necessarily a bad thing - after all, finding wild Level 50 Pokemon in grass when you're only level 20 is as strong an indicator that you're not supposed to be somewhere as a physical roadblock - but it would be nice to see a little more choice and a little more variety in what was done, and a little more challenge in those choices.
But to be honest, I don't think it has to be something new: I'd be perfectly happy if they just upped the difficulty level of what they already have. Challenge Mode in B2W2 was a step in the right direction, but even that was utterly pitiful...and somewhat pointless, given how you had to unlock it. Forcing players to adopt actual strategies in battles rather than just "grind about five levels above your enemy and beat them down with brute force" would help make the Elite Four harder to get to, and probably make the games more fun as well. The problem with Pokemon is that, outside of levelling - which in itself doesn't take very long unless you take it to extremes - you don't really have to work to get anywhere. Making players work would amount to the same thing, I think.
If you talk about streamline travel, that will be the same for many other RPG games as well, not only Pokemon alone. But really, that is the plot device of RPG games, which you (nor the game scenario editor) cannot do much, because without it, the player will not know what to do next.
Unlocking a higher difficulty level is also not something new, which it was done for almost all the Tales of Series games. But, since it is unlockable only at the post-game story, it is also nothing interesting, because when you are at that point in the gameplay, there is nothing much to achieve, that extra difficulty level is then became just an ornament inside the option.
To tell the truth, I'd like GF to learn the gameplay from Final Fantasy series, where you can travel to anywhere in the world up to certain point in the game, but yet have many places you can't go yet, due to story plot, or the level difference such that going to that place will one-hit-KO your party. Then, the middle boss during the plot (Gym leaders if in the case of Pokemon games) have a much higher level and status than normal monster encounterable in the wild, which the players need much more effort than just level-up to defeat.
I'd really hope GF to boost up the levels and power of all sorts of trainer battles (+ legendary Pokemon battle as well), so the situation of me as the player one-hit-KO the Gym Leader by just level-up will gone. Beating the Gym Leader so easily is really not interesting at all, that's why I prefer other RPG games, because even I level-up, the middle boss can still defeat me if I'm not careful, which gives me a challenge that I wanted to accomplish.
But I'm sure that will never happen for sure, because 1) Base states of the Pokemon were dead-set, and it will never receive a major alternation. Even there was a few Pokemon received a stat change in the past, the change was less than 10% of the original BS, so by over-all view, that Pokemon didn't change much from the original. 2) Because Pokemon is there to "catch", so making some Pokemon extremely powerful will just unbalance the player's team if player catch that Pokemon. 3) I found GF quite stubborn about a game's fundamental gameplay system. They may add new features to a new game of the same series, but never do major change to the things that were already done in the past. That is not only for Pokemon, but also for other games developed by GF (though, I do have to say, GF didn't developed a lot of games other than Pokemon).
