Pretty safe bet the mMP will be dual socket with PCIe slots. It'll be easily (if expensively) configured to crush iMP.
There is little to indicate that is safe.
1. There is a huge pricing hole between the top standard configuration of the iMac 27" ( $2299) and the entry point of the iMac Pro ( $4999 ). While there are BTO configures that run up through that hole ( 27" upgrade CPU, 32GB , 1 TB SSD ... $3699 is still an almost $1500 hole. ), that is still primarily just the range of iMacs. There is at least loud moaning and groaning about Mac Pro pricing trends over last 10 years as there is about PCI-e slots.
There are Intel-W options with Apple could hit the $2999 price point the Mac Pro is currently taking up. If the Mac Pro + "Pro Display" is more expensive than the iMac Pro then the fratricide wouldn't be all that significant. The folks who have large sunk costs into displays already are segmented off from iMac Pro ( didn't want all-in-one). There is zero rational in driving those folks to two slot when that is not really their problem.
There is no overall good upside of trying to grandstand the iMac Pro with something even more uniformly more expensive. Too expensive would be a death spiral for the Mac Pro.
[ Dual slots tend to run to expensive even for the Dell/HP/Lenovo folks. That 's why the standard entry configs on those only run with one CPU slot filled (which Apple isn't likely to do at all. ). Moving up to the Intel SP or AMD EYPC doesn't make much sense for large fraction of single user workstation. Folks can make arguments about the corner case (much lower base clock ) single user workload but that isn't gong to sell numbers. Apple's 30% markup piled on top of SP/EYPc relatively high mark up just shrinks the user base. ]
2. No where in Apple's Spring pow-wow about "missteps" did they lament giving up the two CPU socket option. Users wanted bigger single GPUs. Would like to get more timely updates out (two CPU sockets doesn't do that. Especially now on separate product tracks at AMD and Intel. Add on top the Xeon W and Threadripper track more desktop chipsets and update cycles. ). There was nothing there about large for largeness sake being something they missed (there was no "bigger is better").
I think in between the lines in that sessions that was some opening for dealing with specialize workload cards (without jumping through a TB breakout box .) PCI-e slots.... not much of big gain there either. Previous Mac Pros only had 4 slots ( it isn't like Apple is going to reverse and shoot from >4 . ). Probably will be lucky even to get back to 2 open/free standard PCI-e slots. [ Thunderbolt /GPU integration likely will lock down 1 and if Mac Pro matches iMac Pro's 10GbE (with usual Mac Pro dual Ethernet Phys jacks ) and the (at least) dual TB v3 controllers that is likely pragmatically another. Highly likely the Mac Pro has more than one storage device this time. ]
3. More expensive is not what they have the highest need for in differentiation. The iMac Pro has no HDD. The new Mac Pro could ( not necessarily since Apple is SSD focused, but could be a differentiator.... or at least minimally one relatively standard SSD socket/drive connection). The Mac Pro probably would have upgradable RAM; iMac Pro doesn't.