Easy access to all your Pokémon
It makes sense that throughout the series you can only carry a few Pokémon with you at all times. It’s partially the limitations of the hardware the previous entries have appeared, but it also simplifies things, makes battles faster and has you make some hard, firm decisions about who to have with you at all times.
What would really suck is for Sword and Shield to fall back on the old formula of every time you want to switch out your team, you have to go to into a town to a Pokémon center and do it there. That’s a pain and honestly does not encourage you to mix up your party and experiment as often as you would if you could simply change them out at will while you are in the field.
That is an option you should have in Sword and Shield, the ability to change out your party at just about any time outside of battle. Maybe you carry a device (a Pokémon bank, if you will) that connects to any center in the region and you can just transport Pokémon from there. Come up with whatever in-game explanation you want, but just give me the ability to change my party on the fly.