Expected to be fully adopted by 2019, the PCIe 4.0 specs bring 64 GB/s bandwidths in x16 duplex running at 16 GHz, plus improvements such as reduced system latency, lane margining and superior scalability.
https://www.notebookcheck.net/PCI-SIG-releases-PCIe-4-0-specs-teases-PCIe-5-0-performance.260144.0.html