As expected, Linux Mint 18 will be based off the Ubuntu 16.04 LTS release. Linux Mint 18 releases will be based on Ubuntu 16.04 "Xenial Xerus" Long-Term Support packages and supported until 2021.
The new details is that Linux Mint 18 is codenamed "Sarah" and for this release the Linux Mint developers are going to "feature a new look and feel." Linux Mint 18 is expected to be released around May or June of this year. It will be interesting to see what they do to the UI/UX with their Cinnamon Desktop Environment for Linux Mint 18.
These few details were announced this morning via the Linux Mint blog.