it (probably) won't be the same panel as the iMac, and you can't guarantee people will buy one.
iMac maybe not. iMac Pro there is a decent chance. Going with a Ultra Wide panel would allow them more latitude on where to place the CPU and GPU. 3-4 inches to the right ( from rear perspective) and they may be able to squeeze a 'hidden' RAM door back onto it. With some replacement gyrations they might be able to get the soDIMM slots under a door in the middle again.
If they don't care about the RAM door, then can more the CPU and GPU farther apart (which should let them run a bit warmer of increase the overall airflow with bigger inlets. ) . And probably slide in a second 10GbE port if they want. Since left a lot of bandwidth on the floor unused, that's higher utilization of what they are paying for in components anyway ( again spend out thermal load possible with more latitude). If sticking to the "one and only one internal drive" then a second "SAN/NAS" port separate from generic internet port would be useful in more than a few deployments. Or put a "hidden" M.2 drive door so could at least add a working drive. moving the T2 and NAND cards would be easier with more board space (and M.2 would be a smaller door with the bigger output air vent. ) .
If the iMac Pro moves up then they can repurpose the current iMac Pro case for the iMac. Upcoming 'Comet lake' processors are suppose to top out at 10 cores. ( AMD Rzyen is suppose to shoot for 16). Both will probably put the system into the over 100W range when cranking all of the cores hard ... which is the nominal TDP range for the iMac Pro design. iMac Pro's "even bigger" screen helps position it as a more expensive option.
If the iMac is going to jump up into the 6-8 core space, Apple is likely to push the iMac Pro to starting off with 10 cores. ( the price is going up anyway ... at least if stick with Intel. ).
A bit less likely would be for the larger panel to show up at the "big" iMac level with just a bit less. ( less gamut and/or adjustable gamut. Promotion , whatever else they throw on top of the 6k3k panel to tag it with 'Pro'. ), If Apple has volume pricing leverage to push the panel very close to the original 5K panel prices then they could. ( going primarily discrete they have about zero pricing leverage. )
If sold as a typical Apple monitor ( one and only one input and no buttons with software controls in macOS system panel ) the reach is only going to be limited as purely a discrete monitor. On few on non Mac systems are going to buy it. The Mac Pro probably can't drive it by itself either.
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Anyhow, given that it seems unlikely the March event will be Mac Pro day, my guess is 60:40 split between WWDC or April round table. So maximum 80 days and counting...
Given nothing in macOS drops in Dec-Janurary (and even Intel not shooting for March) ... March wasn't likely anyway. Those odds are probably at least backwards. 40:60 and probably closer to 25:75 (WWDC:April).
Even if above (#11362) "May" thing is right, they'd still probably say 'something' in April if not a full reveal.