Linux Mint 18 “Sarah” is the next major Mint version. It is going to be based on the already released Ubuntu 16.04 LTS. The RC should arrive within the next few weeks but so far there’s no public release. However there’s a video on Youtube demonstrating Linux Mint 18 “Alpha”, which is not an official release in any ways, because Mint team only releases RC before the actual release.
I don’t know how the author of this video got hold of the pre-RC ISO but it seems like that this is indeed Linux Mint 18 in the video with the new Mint-y theme based on Arc GTK and Moka icon themes. So go ahead and check out the new artwork of Linux Mint 18.