512GB SD Cards are pretty cheap these days ($50) ... you wonder how long it will take for 1TB cards to get into that price range.
They would alleviate the cart issue quite a bit, even though people hate it you could have just a 32GB cartridge with enough of the game on it to get you up and playing and the rest on SD Card.
Maybe Nintendo can do two SD Card slots for the next Switch, that would help expand storage a lot too. I'd assume probably 128GB on board flash.
More likley much like current Switch games the sizes will be smaller as the game will be more compressed. SD cards willbe bigger. Balances out