The reason AMD kept the same package size and pin count for Threadripper and EPYC isn't just for economics of scale sake, but also for the sake of flexibility to counter Intel's responses. And so they will. For the past couple of years AMD has been doing their best to keep their ability to respond open, so quick and rushed maneuvering from Intel, as witnessed with the announcement of the 18 core HEDT option before Intel is even sure what kind of specifications it will have, will not be able to catch AMD off guard and without the ability to retaliate.
AMD has setup the core wars to be in their favor for the next couple of years. This is information for 2000 series Threadripper coming next year to X399 and X490/X499.
The extra package space available to Threadripper will be utilized next year to create a Tri-Pinnacle Ridge MCM for the platform. They will come in 12 core, 18 core, and 24 core flavors. While they will slot into existing motherboards with a BIOS update, only the extra cores will be of benefit. For the extra I/O afforded by another Pinnacle die (6 channel and 96 PCI-E lanes), motherboards would need to be new. They have the available pins, but they're not setup for it. One of the dies can also be disabled for a 16 core option.
All of the above I confirmed from my sources to be happening. From here on out, this information is still under debate inside AMD for the best course of action, or depends on hitting certain performance targets.
Platform: There is the debate of whether it's worth to create another HEDT platform even higher than X399. In such a case, there would be quad channel 64 lane X490 (X399 refresh), and hexa channel 96 lane X499. These X499 motherboards would be incredibly expensive, but would allow AMD to increase dominance over Intel in the HEDT space in every way, not just core count.
There is also the question of higher TDP headroom, as three Pinnacle Ridge dies would need more headroom than the current X399 is rated for to hit the intended clock ranges.
Performance: Pinnacle Ridge will be higher clocking and more efficient die at the higher end of the clock spectrum, though it will remain about the same efficiency wise in the 3GHz range. Slightly larger, but still under 200mm^2. For this reason, the 18 core variant should clock higher than 16 core Threadripper while staying within X399's TDP limits. I am told to expect 4.4GHz XFR or thereabouts for the most expensive 18 core SKU, with base clocks in the range of 3.8GHz to 4GHz.
For the 24 core SKU, X399's TDP limits will not suffice to run at the higher end of the clocking spectrum. There is the debate of whether to make it exclusive to X499, and potentially X490 if TDP limits are raised to accomodate, or allow it to run on X399/X490 with a reduced clock profile when on these machines (Which can be overriden with overclocking).
Pricing: AMD hopes to remain aggressive in pricing should market conditions allow them. Current targets are 1200$ for the top 18 core, and about 1800$ for 24 cores. These prices are subject to change as the products are far enough away for Intel to have a chance to re-organize their pricing by launch.
Timing: Should all go well, AMD is looking at around early Q2 2018 for 18/12 core on X399, and late Q3 2018 for 24 core Threadripper 2000 parts, with a showing at Computex of X490/X499 and the lineup.