First things first, they should not use the "Wii" name anymore. No Wii U 2, no Super Wii U, or anything related to Wii. Not even the Wii Sports games and Wii Party.
Second, they should use a decent controller that's not strange and unapealling. Just the Wii U Pro Controller and it's fantastic.
The recipe for non-failure.
I dont know about Wii Party, Wii Party U wasn't a major flop. If they dropped the Wii name I could see them keeping the Mii characters and making a Mii Party.
Can't wait for The Zelder Scrolls 3: Breath of The Wild Hunt!