Kotaku confirmed that Wii U will not be receiving any other Assassin's Creed 4 DLC--including multiplayer add-ons.
This isn't the first time Nintendo fans haven't been able to buy DLC off the eShop. Mass Effect 3 for Wii U never received any additional add-on content, for example. There may be some hope for Nintendo fans, however. Assassin's Creed 3 also launched with no DLC support on Wii U--only for Ubisoft to turn around a few months after launch.