I see basically 3 ways to go, and all 3 would likely use a new process shrunk SoC (Mariko) to achieve improvements:
Switch Lite: Same general size, brighter OLED screen, smaller/no bezels, lighter, no fan/passive cooling, significantly increased battery life, modest/no spec increase, $299 with joycons and dock. This would be an iterative replacement for the current Switch and leave room for further iterative replacements down the line.
Switch Mini: Smaller unit, LCD screen, integrated controls (inc. dpad), lighter, emphasis on durability, no fan/passive cooling, modest/no spec increase, different case colors, $199 for just the console but compatible with joycons and separate cradle. This would be a complimentary unit to the $299 Switch, focused more on the kids market and positioned as a more direct 3DS successor.
Switch Deluxe: Same general size, brighter OLED screen, smaller/no bezels, fan/active cooling, modestly increased battery life, significant spec increase (2x+), $299 with joycons and dock. This would be a long term iterative replacement for the current Switch but could also be positioned temporairily as a higher priced alternative ($349-399) alongside the current unit.
Personally though I think we'll be seeing muliple new units over the next several years and I wouldn't be surprised if we got devices similar to all 3 of these.