Koichi Hayashida and Yoshiaki Koizumi, director and producer of the new mario game, conducted an interview with time. In it they said that the upcoming Wii U exclusive Super Mario 3D World will only support local multiplayer.
Core platform Mario games have a mixed history with multiplayer. The first NES game is well noted for the alternating characters multiplayer for two people. Other games have featured multiplayer modes and more recently later entries in the 2d throwback "New Super Mario Brothers" have featured four player on the same screen at the same time multiplayer. Nintendo's efforts in the online arena have long been criticised as lacklustre and many might take this as a sign that such a position has not changed despite claims to the contrary. Futhermore the Wii U's tablet controller had several "alternative screen" demos featuring the tablet as an auxiliary controller with Mario being an example in them.
Is this a dealbreaker for you or does it not matter as much? It should also be noted the Rayman series, now often positioned as the main rival or even superior to the NSMB series, also does not feature online play in the upcoming Rayman Legends or its predecessor "Rayman Origins" but does have an online "challenge mode". Fellow Nintendo title Luigi's Mansion: Dark Moon does feature it as does one time franchise rival Sonic the Hedgehog.