During Josh's talk about the Linux kernel in Fedora, he shared that Fedora 21 will likely ship with the feature-rich Linux 3.16 kernel. However, depending upon the timing of things, it could end up being Linux 3.17. This isn't a huge surprise given Fedora 21's early November release and going into beta in September.
Regardless of what Fedora ships as the official kernel, they'll continue pushing down new kernel versions as stable updates once Fedora 21 has been released. Fedora 21 users will end up seeing Linux 3.17, 3.18, etc. The Linux 3.16 kernel is also what's being targeted by Ubuntu 14.10. Debian 8.0 will also likely use Linux 3.16.
Embedded below is Josh Boyer's presentation for those interested Fedora users.