Nintendo really needs to implement some online co-op (or online multiplayer in general) in these recent Mario games.
New Super Mario Bros. U, the upcoming Super Mario 3D World, any upcoming Mario Party games...they all need it. I mean posting screenshots of me playing onto Miiverse or leaving tips on the overworld map gets old. I rather play just play with a friend online.
Not just Mario either. Any first party Nintendo 2+ player games should have a online option. Nintendo Land & Pikmin 3 would be great if you could play online.