It is way too early to tell, but I expect the Wii U's library to be fairly small by the time its successor comes out, very similar to the N64 and Dreamcast. If I end up being right, some of the later release games (mostly ones that have not come out or been announced yet) will be the harder to find ones, especially third party stuff.
At the moment though, I can't think of any genuinely hard to find Wii U titles. Even Pikmin 3 is relatively easy to find.