It's a fair call to suggest that rev B iMac Pro appears at the same time as a 2019 Modular Mac Pro, but anything longer than an annual refresh of some sort will make professionals very twitchy. Imagine the grousing when a full year passes by with no refresh of any type.
That's common in the Xeon E5 and SP space. Intel hasn't rolled on 12 month upgrades in the server targeted CPUs in ever since they switch over to the "E5" naming system.. The chipsets typically sat on the same iteration for an entire tick/tock cycle.
It is happens to Intel SP (and likely Intel W) in roughly 12 months in late 2018-2019 largely because of two abnormal events. One the Meltdown/Spectre stuff. Two, Intel is stuck on process. It is almost the same exact process tech 14nm++ (maybe 14nm+++ whatever negligible thing that is) and the exact same microarchitecture. The average for E5 was over 12 months. This isn't the desktop/mainstream stuff. It is the sever stuff that goes through longer and deeper QA before release. Few folks really wants yearly updates in the server space. 14-18 for an average range is an expectation that is closer matched to the reality of this product space for Intel. ( big die GPU aren't that much better than that. there is more rebadging and hocus pocus to make it look like it isn't, but it is roughly in the same boat. )
When Intel hits a micro architectural change step in the cycle it is extremely likely going to be longer than 12 months. Shrinking primarily the same implementation down can more easily fit in a 12 month cycle ( presuming that the process shrink works. ). Where they have to extend the stay on a process size for a long time.... Apple may/may not skip some of those if the relatively minor tweaks don't make much difference.
The iMac and iMac Pro aren't going to move at the same rate because they are using substantively different product line in terms of CPU SKU.
There's many things that Apple could do to differentiate between iMac Pro and Modular Mac Pro.
More powerful GPU than the iMac Pro, perhaps even using a traditional PCIe 3.0 x16 card.
They don't really need a more powerful one. One that isn't being substantively underclocked could be a 15-30% boost in performance. Two would be a doubling of performance for apps that scale close to linear. 1.5x for those that scale a bit less than that. Couple that 15% + 50% together and there is a very sizable difference.
More powerful CPU, but I think dual CPU systems might not be an option for Apple.
the iMac's are slightly clocked down but that is a trade-off the Turbo isn't tuned down that much and the base difference is very incremental. if Intel will give a Apple a bigger discount if they buy more of there "custom tuned" ones then those would probably get shared. If just buying in the Xeon W class is good enough then just use the 'regular' W products but 'more powerful" is a bit of an oversell. It isn't going to be a huge cap.
Thermal management though can be a player here. If the Mac Pro has bigger, overcapacity coolers the Mac Pro could run on longer jobs at higher rates even if the even hotter GPU(s) are also running full blast. the Mac Pro could also handle crappier external environments (e.g., being put into a box that is only "good enough" ventilated. )
Internal drive bays - for 2.5" drives
For the new relatively large capacity, but super duper expensive 2.5" SSD drives? For 2.5" HDD and older/much cheaper but significantly slower SSDs. I'm not sure Apple would go there. If looking to put a major capacity gab (at reasonable prices) between an iMac Pro single drive and a Mac Pro with 10's of TB then 3.5" is a gap maker. (adapter goes down to 2.5").
For 2.5" that cap out at 2-4GB there is probably going to be a bet those could get covered by an SSD.
Lots of Thunderbolt 3 ports
More than four TBv3 on any system gets into the point of diminishing returns. More than the iMac Pro won't help. Apple would probably make more folks happier with more than four USB Type A ports ( e.g. add two Type A to the front. or perhaps a Type A and a Type C USB only on front ) than more TBv3 ports.
More than four TBv3 ports as an attempt to get rid of mDP and/or HDMI ports is very bad idea. Trying to get rid of all of the USB Type-A ports for TBv3 is also bluster for no good reason. The iMac Pro isn't lesser because it has four Type-A ... it is probably better off.
More that four in some kind of port pissing match versus an MBP 15" is kind of loopy. The laptop is going to "loose" one to power. So four is already bigger than three.
All within a more cooling powerful solution more able to keep the system quiet too
Again quieter than a iMac Pro shouldn't be a metric. Quieter than a 2010 Mac Pro would be sufficient. (trying to surpass, or even match, the MP 2013 would be also a dubious move. Multiple fans and bigger power curve makes that hard. If throw in a HDD than likely toast on that front. )