Dragon Quest Heroes I & II, a launch title in Japan, is already too large for the internal storage and requires a SD card to download. Seems that the size isn't a hard limit. Still, a game featuring many regions just doesn't sound likely.The Switch can only hold a game as large as 32 GB
Open world Pokemon is a interesting idea, but unlike Zelda, Pokemon needs a more radical change in mechanics to fit into the concept. In Zelda you may be able to beat a boss earlier than expected if you're skilled enough (e.g. playing a second time), while in Pokemon level grinding is required more or less for that.