I sincerely hope that Wii Fit Plus is an actual sequel and not just the same old Wii Fit with Wii Motion Plus support.
How did TP fared in US compared to WW? and either to OOT?
It's hard to say, but Wikipedia puts the worldwide sales as follows:
The Legend of Zelda: Ocarina of Time (7.6 million)
The Legend of Zelda: The Wind Waker (3.07 million)
The Legend of Zelda: Twilight Princess (Wii version) (4.52 million)
The Legend of Zelda: Twilight Princess (GCN version) (1.32 million)
Looks like Twilight Princess sold almost twice as much as The Wind Waker worldwide, so I don't know if Nintendo would be willing to go back to that style.