I have a rather large idea that's been runnin about my head for months now, but anytime I've brought it up to anyone they haven't agreed with me.
It'd be a Wii game, and just recently, I'd thought that you would be able to choose which region you'd start in (although no matter what, you'd eventually be able to visit all of them). You would design your own character and play through a totally new storyline (that would be pretty similar no matter what region you chose). There would be a wider range of starters respective to that region, for example, in Kanto, you could pick between the usual three, Bulbasaur, Charmander, and Squirtle, but also Pokemon like Pikachu, Vulpix, Growlithe, Poliwag, Abra etc. The one that you picked would then be far less common in the wild.
The game play would be dramatically different. You would control your character as you do in a typical, modern Legend of Zelda game. The landscape would be better designed, and we'd be able to fully grasp the beauty of the Pokemon world. Likewise, instead of being randomly pulled into a battle sequence every now and then when running through tall grass, you would be able to see Pokemon around you, and you'd be able to chase them to try and catch them. This would give things like footprints and habitat a much bigger role, as you would be able to hunt and track Pokemon to catch different kinds. I'd also like some kind of real time battling to go with this game, instead of the regular turn based action that we get in the handhelds.
When flying, you'd be able to control your journey like in Guardian Signs, and when your surfing it would also be a similar concept.
I'm still working on it and thinking about it a lot, and I know it's a tall order, but when I think about it in my head it looks amazing.
I'm also in full support of Pokemon Snap 2 and Stadium 3. In Stadium 3, again, as I've said in a different thread, should have some way that they could connect an old Gameboy to the Wii and transfer their old Gen I and II Pokemon into the game, where they'd be converted into the Gen IV data type, and so you'd be able to use your old Pokemon.
It's not really a spin-off, but I think that's an amazing idea. The whole footprint idea would also make Pokemon that float rarer, like Duskull.