There will always be more Pokemon to be invented and explored, and I doubt Arceus, Palkia and Dialga will be the last legendaries.
And there are always possibilities for other narratives - perhaps Arceus wasn't the original God, maybe Arceus replaced a far more ancient being, maybe a God of destruction could be awakened by Arceus' presence, maybe there are Pokemon from alternate realities, from different planets, from the virtual world, Pokemon that only exist at an atomic level...
...the possibilities are only limited by the developers' imaginations.
Probably too soon for a Ruby/Sapphire remake. An all region game would mean Nintendo wouldn't be able to sell (at least) four carts separately - I'd imagine they'd rather keep the games separate for the sake of revenue/profit.
Yeah, yeah, that's true, especially the very last paragraph. Most of this is for profit. None of the gameplay matters as much, just as long as the elements are "new" and "different," even barely.