Hello! I recently bought a 3TB HDD with a separate power supply to use with my Wii U instead of the SSD that I had been using before. This was because the SSD was prone to disc-read errors, and from what I could gather this was probably an issue...
I played Yoshi's Woolly World with a friend once and had a great time with it (only 2 players total though), other than that I would recommend NSMBU or Super Mario 3D World because they have great co-op. NSMBU also has a huge mod called Cloudy...
I am actually having this same issue on Mario Party 6 & 7,
I was able to get Mario Parties 4 & 5 to work in widescreen, but not 6 & 7.
curious to see if this issue ever got resolved