well they can easily do cpu changes on the imac pro. Not so much video.
In the context of 'dust up' between Apple and Nvidia. No recent break on the drivers on that front points to that not being pragmatic at this point as a near-intermediate term option.
There is a "drop in" , CPU socket compatible solution coming (Cascade Lake derivative) , but there isn't much of a change in the context of Apple's iMac Pro focus.
i. Meltdown and Spectre fixes ( Apple will probably be receptive to the incremental speed bump and fewer band-aids need to add macOS. But still several macOS workarounds required so not revolutionary better. ).
ii.. Some "AI" machine learning VNNI instructions (which Apple libraries probably won't use in 2019-2020 since focused work on Apple's engine and GPUs. )
iii. Optane DIMMs . ( Again requires an OS and libraries changes to macOS . Is Apple going to do those)
iv. Some modest clock bumps ( Apple is already running older Xeon W at reduced clocks in iMac Pro ... chips that clock even higher than the clocks Apple didn't want to use are useful how? )
i & iv would be useful for a new "greenfield" Mac Pro. Less legacy Meltdown/Spectre kludges have to deal with over the machine's service lifetime the better. The Mac Pro also probably won't be "down clocking" the W offerings (i.e., designed to substantively larger thermal envelope). It would makes sense to start there. That there are a decent number of cloud hosting folks with current Mac Pro who will probably throw new ones in also in a VM centric context means that 'i.' issue is incrementally even higher utility.
Cooper Lake derivative which uses the next socket is due toward end of 2019. If move the iMac Pro to that then could goose two updates out of that with no substantive logic board changes. Cooper Lake will have
i. some clocking changes. May be able to do more incrementally improved dynamic clocking. Or more problematical a signifcant bump up in TDP range. ( which means Apple will need some new cooling sytsem refactoring to do).
ii. more AI stuff. (bfloat16) which Google's Tensor can leverage.
iii. more DIMMs support options. (which Apple will likely ignore on the iMac Pro )
iv. PCH chipset updates to go along with new Socket.
Late 2019 there will probably be some new options for the iMac Pro in GPU space.
If Apple is trying to use the same team to do both Mac Pro and iMac Pro updates then I easily see them assigned in most of 2018 for Mac Pro ( Cascade lake to catch the security and timeline of late 2018) and then shifted to a iMac Pro revision/upgrade for Cooper Lake and new socket change on a ramp from 2H 2018 up to full time 2019. Then flip again in 2020 back to Mac Pro to either pick up Cooper Lake or Ice Lake depending if Intel has hiccuped on 10nm yet again. ) Any new socket/PCH quirks tackled in the imac Pro board folded into the Mac Pro board.
Even if Apple shifts the iMacs onto 6 cores in 1H 2019 they can probably float the update of the iMac Pro into late 2019 without major problems ( grumbling sure but not major problems ).
Now the new mac pro needs to have video on some kind of card so that it can be upgraded. MXM cards where a part of that idea but the cooling part mad that not really take off.
MXM isn't really designed for heavy duty coolers. If in laptop range of TDP then fine. But if want to go up into the mid-upper range of desktop TDP then not so optimal. Especially if trying to keep noise at below average levels.
Some of the characteristics ( putting video back into the logic board) and no external edge connectors are on the right track, but Mac Pro probably is a bit too big for that if Apple shifts to deskside focus. If they are still laser focused on literal desktop then sure. Painting themselves into just a broader corner and oddball overlap with the iMac Pro.
P.S. Apple should be able to upgrade the iMac Pro and the Mac Pro at approximately the same time. However, their track record on being able to walk and chew gum at the same time over the last 5-6 years is horrible. For whatever reason they have decided that isn't a priority so these two will probably be single tracked with only modest overlapping pipeline work done. Even with Apple adding a single, leapfrogging track that is active year round , ever year , that is still way better than Rip van Winkle mode where they go completely to sleep for years at a time. That's progress.