This how my mind works late at night.
Okay, so making a sequel to a video game is sort of like playing Jenga. For those who don't know, it's a game where you have a tower of wooden blocks, and you have to take a block out of the tower and place it on top. The object is to see how high you can build up the structure without it falling over.
So in video games, you start with a foundation. A solid game that sold well enough to justify the time and resources required to continue the franchise. The object is to keep the foundation, or the core gameplay of the original game, as steady as possibly, while adding new content to it. (putting the blocks that you took out of the original tower on top) It's all about balance.
Let's say the original gameplay was glitchy. Well, we can take that block out and add it to the top to make it higher. The graphics need updating. A new storyline is added. We're good so far. But the problems start occurring when you disrupt the structural integrity of the original, and take too many blocks from the wrong places. The tower collapses, and the game bombs. Or, if you add the new blocks to the wrong places, the foundation can't support it, and it overbalances and falls over again. Most times, it's a combination of both. Look at the Sonic franchise. They changed the original fast-paced gameplay too much that the foundation was no longer stable. Then on top of that, the glitchy controls and camera, bad storylines, and cheesy dialogue were like bricks added in the wrong spots on top of the tower. Eventually, these flaws start outweighing the good points, and tower falls down.
Again, it's all about balance - changing enough of the foundation while still adding enough new content to make it fresh, and worth buying again.
"But SWE3tMadness," you might say, "in real life, you don't have a limited number of blocks. You can keep the core gameplay intact and only add on new content!" Ideally, yes, you can do this, and often this is how the best game sequels are made, for example, SSBM. Take the core gameplay and frantic, multiplayer action, and then add new moves, new stages, and new single-player modes. Voila! You have the best-selling game on the Gamecube, and arguably one of the best multiplayer titles ever produced.
However, there is still a limit. Imagine a Jenga tower five stories tall. It'd collapse in a hurry, right? If you add too much new stuff, and don't change the foundation at all, eventually it still won't be able to support the extra weight. Eventually, you have to change the foundation.
This is where Pokčmon franchise is starting to fail. Everyone loves the Gold and Silver versions because they added just enough new content while keeping the original addictive gameplay intact. It worked incredibly well. So Nintendo decided to keep on doing it. However, now the more recent games have so much new content that they're on the verge of over-saturation. While there are new areas to explore, new moves, items, pokčmon, trainers, etc, the basic foundation of "Capture pokčmon, train pokčmon, battle pokčmon, lather, rinse, repeat, etc" hasn't changed. The series has grown with so much new stuff to do that the core can't support it anymore. Nintendo really needs to address this problem before they just slap on more content to the next game.
They've tried too. The addition of Contests allowed us to apply a completely different strategy for building and training a party, and the Wi-Fi multiplayer from Pearl and Diamond allows you to take advantage of a very deep system of battling with other Pokčfanatics all over the world. But these additions still boil down to only new content. Again, Nintendo needs to change the core of the franchise. While this may sounds like blasphemy to some people who say "If it 'aint broke, don't fix it", the system isn't really broken, but is no longer sufficient to support the continual addition of content in each successive sequel.
So how can they fix it? I've always said that the world of Pokčmon would be perfect for an MMO. Not everybody in WoW goes off to find epic lewtz. Some just like exploring, trading, working the market, and interacting in an online community. Just like that, not everyone in the Pokčmon world necessarily wants to raise level 100 maximized killing machines. Give us a chance to create our own character and decide what we want to do in the world. Pokčmon Ranger and Mystery Dungeon were good ideas to expand the series in this way, but Nintendo treated them as only spin-offs to the franchise, and still pours most of their resources into creating the traditional sequels that are stagnating.
In conclusion, if you've made it this far through my post, congratulations. But in all seriousness, as much as I love the Pokčmon franchise, Nintendo desperately needs to rework the foundation of the traditional hand-held RPG’s in order to keep the franchise viable