So this week the Dev team is all together or 5 days of coding to get FreePBX 13 going. The whole focus of 13 is a consistent GUI and more responsive and the continuing of building out the FreePBX UCP that was introduced into FreePBX 12.
We will keep everyone updated with a roadmap that can be found here.
FreePBX 13 has nothing to do with the OS its just a front end. As far as the FreePBX Distro we have not laid out a timeline on working with EL7 at this time as EL6 is rock solid and not going anywhere anytime soon.