There are so many games that haven't come to the Wii U that would have been great, one of the most notable games not to come to the Wii U is Grand Theft Auto V, in my personal opinion the game would have had the potential to be the best it could be on the Wii U but was never given the chance to shine. Out of interest Rockstar did ask a couple of years back in people would be interested in seeing a Wii U release for the game, still nothing came out of it, however a rumor has come to light that at one point Grand Theft Auto V was being prototyped for Wii U.
Unseen64's Liam Robertson is the one who brings us this rumor and is well known for finding out about cancelled games and development secrets. This does seem to add some credibility to this as Robertson was in touch with "a reliable source" who claims that the prototype “was an earlier version of GTA V.” It was apparently a brief period when this took place. Further details are unknown however, such as why the game never went further.
I would have loved to have this game for my Wii U but like many I guess we will just have to dream about what could have been.