There's enough to make a 20 year long anime, so yeah, I'd say there is enough to make a Pokemon open world game, easily. It's not like they don't already have a rich world full of cute, quirky and irreverent stuff. None of it has a true light shone on it, but then thats why it would work in a game where you bring your own playful interpritations.
So yeah, Pokemon would make an excellent open world game
but, for many reasons, valid and not, they won't make it any time soon.