A robust online component
For a lot of people who play Pokémon, the actual campaign is the least important part of the game. It’s the trading and battling you can do with other people that is the biggest appeal. There is no doubt this will be a big component of Sword and Shield on the Nintendo Switch, but there is the opportunity for so much more.
I’m not sure how realistic it is, but being able to connect and trade with owners of any previous entries seems like a cool idea. The most obvious idea would be Let’s Go as that’s already a previous entry on the Nintendo Switch, but some kind of interactivity with Pokémon Go would be great, and if at all possible, as well as the Nintendo 3DS titles like Sun and Moon. Being able to use the Pokémon Bank seems like a must for real hardcore players, but I’m not positive how feasible that is (obviously would be cool to have, though).
That is just the tip of the iceberg, however. You could have tournaments, world events, special missions for multiple players, limited time exclusive legendary hunts, team battles, special challenge areas only available online. There’s a lot you could do to make Sword and Shield the definitive online experience for fans, so I hope Nintendo delivers on that.