I say Sunshine 2. A sequel would be amazing. All the water effects and the particles in HD would look fantastic. Also I like how Sunshine had more exploration while Galaxy was pretty linear for the most part. The fact you could see the other areas in Sunshine was a great touch. I like Galaxy but theres already 2 of them and I love FLUDD. The music in Galaxy was beautiful but I'd take gameplay over music anyday. Also Sunshine had that awesome vacation music. They should have 2 players. Mario and Luigi. 4 would be too hectic for a full 3D Mario game. I'd also like a Sunshine remake but I'd rather have a sequel. Which would you have?
EDIT: To all the people saying something new, i'm saying in between these 2 games. I'd like something new instead as well, but don't say neither because its off topic. Between Sunshine 2 or Galaxy 3. not Sunshine 2, Galaxy 3, 64 2, or something new.
My Wii U exclusives:
Captain Toad: Treasure Tracker, Donkey Kong Country: Tropical Freeze, LEGO City: Undercover, Mario Kart 8, New Super Luigi U, New Super Mario Bros. U, Nintendo Land, Pikmin 3, Scribblenauts: Unlimited, SiNG Party, Sonic Lost World, Super Mario 3D World, Super Smash Bros. for Wii U, and ZombiU