Speculations, I guess, have an air of reasonable prediction. With the series' track record, its entirely likely that as little as the name could in effect change.
As far as reasonable expectations go, I'd like to see some (of course, not game-breaking) compatibility with the new DSi. Maybe a semi-customizable sound track or Mii support. video chat on the bottom screen over wi-fi would be kind of cool, if slightly superfluous. Which also ties in with that I'd like for the next generation to be on the DS again... not the DS2, but the DS.
As far as more outlandish options... Well,
-Custom pallete (trainer, menus) would be cool and easy to implement. I'd absolutely adore the ability to change the colour of the menues, and 5-8 choices of sprites for male and female would be neither difficult nor detrimental to gameplay. If you add in pallete recolours (easy to make and even easier to store on the game card), there could be 40+ different looks for the main characters.
-More intricate plot, and no, I don't think that means that a "dark" story is necesarry. I think a story can be just as interesting without any legitamately adult themes, but feeling worried for characters in situations of peril would do a world of good for the game. The key to this is character devellopment, not "dark themes", which leads to caring for characters; this is why the animé and manga can be so attractive to those who want a more in depth look at the pokémon world.
-Cutscenes. Wether they're sprites, 3-d, or fmv animé, its always great to get a closer, cinematic look at the world of pokémon. Imagine even a brief, 3-4 second HQ animated shot of the protagonist stepping for the first time out of their hovel and into the world beyond, or emerging from the caverns of Mt. Moon and confidently setting his or her eyes on the Cerulean Gym who's badge would soon be his.
-ERRADICATION OF HMs. Or at least, as we know them: have each pokémon have one or two sub-ordinate HM slots for moves that can only be used outside of battle. Personally, I think this would add a lot to the game for both battlers and just players looking for immersion and who don't like to have to switch bidoofs they never wanted to catch in the first place in and out of their party just to access areas of the game.
-As far as gameplay is concerned, I'm all for those little extra things that really get the player involved in the world of Pokémon. I'm talking about the animations of Crystal, or how in DP gym leaders will give their last pokémon words of encouragement instead of sitting idly by. I think that making Gym challenges more true to the show (perhaps similar to how the Battle Stage is handled in Platinum, with a large stadium and such prefacing every round) and other more characteristic properties. I'd like to run into more characters like the Unnamed Executive we saved from Team Magma/Aqua in R/S/E, or the Cutter who's Farfetch'd we rescued in the forest in G/S/C.
-On the technical end, I'd like for battles to stay largely the same, with introduction of the normal elements. People have mentioned things like a 60-100% freeze which I think would be cool. I'd like to see more attention given to underused pokémon, trying to expand the movepools of everything to a somewhat respectable level. I really like stuff like Charge Beam on Empoleon or Flamethrower on Manectric: sensicle, maybe not from a player's standpoint at first, but given the features of the creatures and given that these are inhabitants of a world where they behave realistically. Heat Wave for flying types was a good start. Now, Seed bomb for those same flyers. Flamewheel on Donphan and Hitmontop. Pretty elementary here.
I think thats just about it. Damn, if Gamefreak made a game like this... I'd buy TEN copies.
