So looking through some of the comments that were made on the gameplay footage video of Heart Gold and Soul Silver I put up on youtube got me thinking and I decided to post a little more detailed speculation of my own on the new games, with due credit to all the users who posted on youtube for brainstorming help.
So, first off there's numerous suggestions in Diamond and Pearl, and even in Ruby, Sapphire, Fire Red, and Leaf Green that time has past since the original GSC versions were released, in the pokemon world.
Basically what I mean is that if there was one giant time line where the events in the game took place during a certain year or what not, diamond, pearl and platinum is in the future when compared with the events of GSC. Which means that a true remake isn't going to be possible. They've referenced past events of GSC such as the lake of rage Gyarados, and obviously team rocket's interruptions there. All that means is that Heart Gold and Soul Silver will actually have to undergo significant cleaning to ensure that they are, storywise, compatible with what's already been established by the 3rd and 4th generation of Pokemon games.
And of course, as we all know from the couple seconds of footage, that Chikorita was following Gold around. Now whether this means that the first Pokemon in your team will follow you around always, or just your starter will follow you around, or perhaps there's going to be a size limit where after your starter evolves, it spends it's time in it's pokeball instead of walking alongside you, who knows! :O But it's clear that a certain Pokemon Yellow element is being reintroduced through Heart Gold and Soul Silver.
I also noticed a couple comments about new pokemon in the GS remakes, and I would agree that there will not be any new Pokemon, fifth generation Pokemon, in Heart Gold and Soul Silver. They are remakes afterall, and if the FRLG remakes were any indication, the best we can hope for is that some legendary gets a new alternate form, as Deoxys, Shaymin, and Giratina have gotten. New alternate forms is pure speculation.
Regardless though, to both ensure that these remakes are brought up to spec with the standards set by Diamond, Pearl, and Platinum, some serious work will need to be done. After all, when FRLG were made, it was for the game boy advance, and thus wasn't too far away from the original gameboy system. With the DSi now out, Nintendo may even incorporate DSi compatibility into Heart Gold and Soul Silver in an attempt to promote that handheld console. All this means is that the gap between the original Gold and Silver, and the new Heart Gold and Soul Silver must be significant, and the gap will be greater then that between Red and Blue, and Fire Red and Leaf Green.
Also, with the remaking of Gold and Silver, Nintendo has an unprecedented and unique opportunity to bring former Pokemon players back into the fold, and to get them to once again buy and play the games that they may have stopped playing years ago. In order not to waste such a golden opportunity to capatilize on what will make them even richer, they HAVE to make Heart Gold and Soul Silver nothing short of amazing. As if that weren't incentive enough, there's the fact that Platinum set the bar high as one of the best pokemon games in recent history, which means to top it, Heart Gold and Soul Silver have to be even better. Also, because there aren't going to be any new pokemon in this game, and the GS remakes will not help you complete your pokedex, it HAS to be heavy on features and additional plot/storyline development.