In the meantime, Apple doesn't sell anymore the 4K and 5K LG displays but keeps selling the tMP... I hope it's not connected, but it subconsciously gives the idea that the mMP is not so close by. Strange, since I wonder what other macs could run the 6K display Apple may come out with.
As noted in previous post, it looks as though Apple has filled in a replacement for the 4K UltraFine, although a bit 'less' 4K than what is being replaced. Less pixel density (and probably less BOM cost) is swapped for it being a true Thunderbolt 3 docking station display ( as opposed to the USB-C DP alteratinve mode before. )
As far as what could drive a 6k3K display besides a Mac Pro....
Probably everything on the LG Utlrafine 5K required Mac system list https://support.apple.com/en-us/HT207448
Perhaps a firmware tweak to the systems to recognize 6240×2880 as 'standard' resolution option. ( It would be fed via two DisplayPort 1.2 streams just like the 5K ; only bandwiith would be closer to the max available. ). What is the framebuffer max needs to move up a bit and some low level stuff tweaked. Don't really need any more "grunt"/"horsepower" to simply just drive the display mostly in 2D mode.
If that new 24" is the replacement for the 21.5" and the 6k3k is the 5k replacement, then the recent large draw down in 5K inventory means Apple could release the 6k3K at WWDC along with the software/firmware updates ( perhaps buried in 10.15 or one last 10.14.x update. ) .
The new "Monitor" is extremely likely going to be a Thunderbolt docking station display. Hooking up to laptops and the Mini is also in the scope of the target market. In fact, the non-"Mac Pro" set of Macs would probably be at least as large a set of folks as hooking up to new Mac Pro in terms of unit sales. Apple is highly unlikely to making a monitor that only the Mac Pro can drive. The volume on the panel would likely be below their "minimum run rate" threshold of being interesting.
It wouldn't be surprising if they flipped the iMac Pro to the new 6k3K panel to get the panel volume up over the threshold. Docking station , Mac Pro , and iMac Pro would probably get to "high enough". Apple may not do all of that in just one step, but shipping the monitor out in June in advance of new Mac Pro ( and/or iMac Pro changes ) could work but a slower volume ramp. ( as a new tech panel and backlight that may not be so bad. )
P.S. I don't think that new 24" panel is destine for the "Retina" iMacs. Perhaps Apple is going to tweak the "smaller" iMac line up and put that mainstream panel in the "edu"/"entry" iMac and dump that last old school classic density. And move the 21.5" to a new panel that is more upscale ( if keeping the common iMac body frame at that size. ), The smaller iMac could iterate at the end of the year. ( doesn't have to match in size right away).
It is a mainstream panel so Apple could just get their volume discount from the maturity and market volume and just keep their 21.5" size.