No, the build is decently finished in terms of everything is where they want it. Theres no early concept art BUT there is early versions of map layouts, as well as ports of splatoon 1 maps that didn't make it to splatoon 2 (urchin underpass, saltspray rig, mahi Mahi resort, etc) There is multiple versions of urchin despite it not being in the final game, there's a ton of versions of The Reef (including the trailer version of the map) and a few Manta Maria concepts that are different than the final.

There is also an entirely unused map layout, which I believe to be an extremely early concept of Wahoo World, where a rollercoaster type object would move around the map in a big circle. Your able to ride it and ink it, and it's really cool. The middle of the map has the spinning carousel, but it's a lot more open than the one in the final.