Forgive me if I am being redundant, but the currently shipping Intel Xeon E3-1280v6 has a 72 watt TDP, which will work fine in the iMac's slim chassis. The E3-1280v6 is based on Kaby Lake and uses an LGA 1151 socket, the same socket used by existing Skylake and Kaby Lake desktop CPUs.
Comparing shipping Kaby Lake CPUs, the E3-1280v6 (3.6GHz) looks like it's a slightly higher clocked version of the Core i7-7700 (3.9GHz), which accounts for the difference in the TDP (72w vs 65w). The Xeon omits an iGPU (HD Graphics 630) but adds ECC DRAM support. The recommended customer price for the Xeon is $612.00 USD, which is roughly double the cost of the Core i7-7700. Double the cost seems like an awfully high premium for a CPU that offers the same core count (4c/8t) and I expect to see that difference reflected in Apple's BTO options for this iMac.
The caveat in all this is that the CPU that Apple is rumored to be using (E3-1285v6) is technically unreleased and none of us know it's exact specifications. However, if I look at Intel's ARK, I can't see Intel doing much more than clocking the existing 1280v6 part a bit higher given that Apple is already using 88w-91w TDP CPUs in the current iMac Retina 5K. All Xeon E3-12xxv6 CPUs are 4c, so don't expect more than 4 cores in a "server-grade" iMac.
Apple will most likely use the existing C236 chipset that was released in Q4 of 2015 for this iMac, simply because it's the really the only option at this point for Apple and it will most likely be what they use in the next Mac Pro (2018?) since it will support both Kaby Lake Desktop (Core i5/i7-7x00) and Kaby Lake Server (Xeon E3-12xxv5/v6) CPUs. In fact, I can see Apple ditching the Z170/Z270 chipsets in the iMac to simplify their lineup as ECC support is an optional feature.
All in all, I think Pro users will be disappointed if Apple ships an iMac "Pro" with a Xeon E3-12xxv6 as most Pro users that would consider the iMac for production will be disappointed that Apple isn't giving them a 6-core or 8-core option, especially considering that Intel sells two 6-core enthusiast CPUs (i7-6800K and i7-6850K) at a lower or similar price point (6800K - $420.00USD, 6850K - $620.00USD). Of course these CPUs have a 140w TDP and use the X99 chipset, which are most likely going to be reserved for the new Mac Pro when it ships...sometime in the future.
Well, at least we should see DDR4 and Optane Memory when the new iMac ships. The other big question, and where Apple will have a chance to redeem themselves will be what GPU they end up using. The AMD Radeon RX580 and RX570 series have officially been launched, so there's a glimmer of hope we'll get at least a 4GB GPU base and a BTO option to 8GB of GDDR5 VRAM.
Hopefully, Apple has a bit more in store for the iMac Pro...time will tell, I guess.