Pylons

The Xeon E3 v6 series uses socket 1151 so it tops out at 4 cores. Apple will not make a Mac Pro using this CPU as a 4-core entry model because then they would have to design a completely different motherboard for the models with more than 4 cores.
In fact Intel always launches their consumer CPUs earlier than the enterprise CPUs, and the Xeon E5 v4 was released just in March, and v5 (Skylake-EP) is rumoured for 1H 2017 (see e.g. http://wccftech.com/intel-skylake-e-lga-3647-hexa-channel-memory/) with a new socket LGA 3647. Jumping to v6 but being limited to 4 cores makes no sense.
Releasing an updated Mac Pro any time before 1H 2017 is probably not likely, since they would have to make a major redesign for the new socket in 1H 2017 anyway.

The current Mac Pro is a very unusual design choice because the space constraints limits the setup to one CPU. This would mean that the CPU model doesn't have to have  support for dual CPUs. Apple uses E5-1xxx v2 series for the lower end but Intel probably figures users with high performance requirements may want to use dual-CPU machines so they limited the E5-1xxx v2 to 8 cores, forcing Apple to use the E5-2xxx v2 series for the 10 and 12-core models.
The E5-1xxx v4 still has max 8 cores, while the E5-2xxx v4 goes all the way up to 22 cores. E5-2xxx v5 is rumoured to have up to 26 cores!
(It will also support 6 channels of DDR4 memory, which is completely unnecessary in the Mac Pro form factor.)

It will be interesting to see what 2017 brings.