Yes, the Wii needs another Wii Sports and Wii Fit, because high quality is too rare in these categories, especially when you account for the amount of people who are interested in these genres.
If third parties were capable of filling the void that Nintendo left, then the system's sales would still be fine. Nintendo can't turn their backs on their audience and tell them to play some trash (which is what most third party software is). It doesn't and cannot work like that.
There wasn't even a need for sequels as those games are still selling strong today. Nintendo should've continued to release new disrupting games. An RPG that focuses on local multiplayer and co-op featuring Mii's would have been the best thing on earth. We're regularly having gaming parties here and everyone has been craving for such a game. A full blown Karaoke game made by Nintendo would've filled a big void as well. "We Sing" is selling quite well in Europe but it can't compete with Nintendo quality. And of course New Legend of Zelda.