PCI-SIG have announced PCIe 4.0 is on the horizon, with up to 16GT/s or just a hair under 32GB/s transfer rate on a 16x slot.
The new standard (PDF) will also allow devices to use more power, the draw from the slot remains at 75W but external power should be able to exceed 225W without exceeding the specs. This could mean GPUs can continue to emphasize performance over power efficiency, which could lead to some interesting products. There is no specific date for any products to arrive, this announcement is technically for revision 0.9, so it is a not quite ready for prime time.
