Plenty of Pokemon Sun/Moon concept art
In Japan, the Pokemon Sun and Pokemon Moon Alola Art Book was a pre-order incentive. Official illustrations are included inside, along with special pictures of concept art. These images show characters, locations, and more.
Here’s a look at the various concept art:
I believe this art was also included in the Pokemon Sun and Pokemon Moon: Official Strategy Guide Collector’s Vault. The images may have been a part of the standard/collector’s guides as well, but I can’t say for certain as I don’t have access to those.