Pokemon will never be mature. Nintendo knows right now that Pokemon is something that the kid market enjoys in the US, and in Japan a larger market enjoys it as well. To change the game too drastically would risk losing the market share and audience it already has.
Pokemon is a brilliant game. The whole concept of collecting and battling monsters is already pretty mature. The only thing that hurts the game is the cute monster designs, which are not going to change because they are engrained into the magic of the game.
I often wonder about a 3D Pokemon game. It seems like such a great idea. Throw in online play and have a massive world to play in and capture monsters. You could even add monsters to the server to collect and it would be an endless game.
The problem with this is production costs. Which is why Nintendo never went into online gaming yet. For quality control of a project this big it would have to maintain complete server control. Costing bandwidth and money...money that it is not postitive it will earn from the game and its target market.
Next, you realize that the design of this game is also going to be expensive. Yes Nintendo has made most of the Pokemon into 3D monsters for a battle game, but the models are basic, and Nintendo hasn't spent time to build a huge world to place the monsters in, let alone update all the designs to look and belong in a 3D world. This is a much larger process than anyone could imagine, because Nintendo not only needs to update the 300 or so monsters, and make new ones, it has to create a believable world and enviroments for this animals to live, and also potentially interact with eachother in the wild.
While Nintendo has been able to keep the focus strictly on the battle and RPG element in the 2D games, most of this is because you don't have to show the animals in the 2D world before you battle them. As well the create of minimal animation for 2D graphics makes the Pokemon series easier to create in the current form.
I believe Nintendo will do another half sequel where they update the series just enough, and add just enough DS support to make the game new and fresh again, but it isn't going to be the revolutionary Pokemon game everyone wants.
I would like to be proven wrong, but I just don't see the benefits of creating the game we want to see, at least from a business perspective.