Maybe I would, but it depends. I don't have a Wii U and I don't plan on getting one, so I don't think I would buy it just for Animal Crossing. But it really depends on how much better the game is than New Leaf. If it was really that good and I wanted it, I would probably just wait to buy it during a holiday, such as my birthday or Christmas.
It would be a really cool idea, but I'm wondering: it would probably be a few months before Nintendo even announced the idea, and then a few more years before they finished the game. By then, wouldn't they have created maybe a few more gaming systems, and the Wii U wouldn't be so popular? It's sort of how they don't really make original Nintendo DS games anymore; at least, the new games don't show up as commercials like they do for the 3DS and Wii U.
So I would guess that Nintendo would create a new device slightly prior to creating a new Animal Crossing game, like how they did with New Leaf.