There are several upgrades that are available today/within the next month for the Mac Pro.
For the CPU, Apple would upgrade to new Haswell based Xeon chips. These would yield a small bump in performance ~5% and similar frequencies as to what is in the Mac Pro today.
GPUs are more interesting. AMD is expected to release new "Fiji" chips within a month. Since all the information on these chips is rumor based, its unclear whether this would make it into a Mac Pro. Otherwise, previous generation "Hawaii" chips are available and would be an upgrade over the "Tahiti" chips that are in the D700. Either of these options would be a significant increase in speed compared to the current offering.
There have been significant upgrades in SSDs since the release of the Mac Pro. Even looking at what is in the new 15" macbook pro, this would be a big increase in speed.
Longer term, nothing will change in this picture until early 2016 when Intel releases Broadwell based Xeon processors. Anyone who mentions Intel skylake based processors in 2015 for the Mac Pro are wrong, since these will only be consumer chips, i.e. for laptops and consumer desktops like the iMac. Skylake based Xeons won't be released until most likely 2017. GPUs will likely not change much until mid 2016, when both AMD and Nvidia release the next generation of graphics processors on a smaller transistor size.
Also, the schedule could be influenced by the release of thunderbolt 3 and Displayport 1.3 if Apple wants to release a retina thunderbolt display they may have to wait until late 2015 to do so. Essentially, Apple needs a higher bandwidth connection between the Mac Pro and the display to be able to handle a 5k resolution. A thunderbolt 3 controller is rumored to be released around the time of skylake, but this controller could be used in a Mac Pro without skylake. Intel is releasing some information about thunderbolt 3 next week, so maybe we can guess more information then about timelines.