I think the "next Switch" is just going to end up being a hardware revision. Either a Switch XL with a larger screen, or a Switch Mini with integrated controls which brings the price way down. I could see them going in either direction.
As for a true next-gen leap, I've had this idea for a while now. They should revisit the Wii U idea of two-screen gameplay. Only this time it's the handheld streaming to the TV (or dock) instead of the console streaming to the handheld. Basically, your Switch would be like a Wii U gamepad that you could take with you anywhere. It would have everything that makes the Switch great right now, as well as the gameplay advantages that the Wii U brought to the table.