The colourful Wii U title Runbow has recently been listed on the Amazon US website with a release date of September 6, 2016. The title has previously only been available via the Wii U eShop, and this recent listing seems to be indicative of a physical release.
Alongside the Wii U physical version, a Nintendo 3DS version has also popped up with the same release date. The information on the listings show that the physical copies will be published by Nighthawk Interactive.
Prior to the appearance of these two listings on Amazon, it has not been announced that physical copies were in the works. We will keep you posted with further information as soon as it comes to light.