As you may know, the Linux Mint developers have decided to adopt only the LTS versions of Ubuntu as code-base and focus on implementing Linux Mint-specific fixes and enhancements.
According to the Linux Mint Monthly News blog post from December 2015, 2016 will be an interesting year for Linux Mint.
The current year will bring the XFCE and KDE images of Linux Mint 17.3 Rosa and after that, the development for Linux Mint 18 will start.
Linux Mint 18, based on Ubuntu 16.04 Xenial Xerus will bring new features, systemd as the default init system, GTK+3 support, an enhanced user interface and some stability changes.
Also worth mentioning, the Linux Mint Team will release an official upgrade path between Linux Mint 17.x and Linux Mint 18, so that the users don’t have to reinstall everything.