This shouldn't come as a big surprise since Fedora usually always ships with the latest stable GNU Compiler Collection release. GCC 7.1, the first stable GCC 7 version, should be out in late March or early April while it's just about in the final state of development and could allow GCC 7 being added to F26/Rawhide very soon.
The proposed change with migration plan for GCC 7 is outlined and will likely be approved. The goal would be to rebuild all F26 packages with GCC7, but if that doesn't happen, just the key packages would be rebuilt.
Other recent proposed changes for Fedora 26 include Transdiff and automated AMI test and release support for the Amazon EC2 Cloud.
A current list of approved changes for Fedora 26 can be found via this Wiki page. Fedora 26 is currently planned for release on 6 June.