I too would want a thicker case with a more industrial look from behind. Think the back ringed with with Allen or Torx bolts. I'd like the look plus the it would make it much easier to service or upgrade. I'd like to get away from aluminum. Personally I'd like bronze or high carbon steel so it develops a nice patina. Cast iron would also be cool. The Apple logo should be the old rainbow. Actually I'd use the logo, badge and font from the Apple II era. The stand would have height, tilt and pivot adjustment. Plus unbolt using a standard 100mm x 100mm VESA mount.
GPU: Speaking of thickness I'd want it to be able to handle a full sized double slot blower style GPU fitted sideways for easy upgrades. Internally using multiple DP connectors to the GPU to allow for easy upgrades, running the internal display and throughput to Thunderbolt connectors. Plus dual 6+2 pin PCIe power cables.
SSD: Two standard M.2 connectors PCIe 4.0 x4. They can have the T2 chip. Make it something you can disable in recovery mode. If you don't care about the secure enclave and just want something you can upgrade. Four connectors in iMac Pro
HDD: None standard. Bays for one to three 3.5" HDD depending on screen size.
USB: Use the new USB 4 standard or at least USB 3.2 Gen 2x2 w/ Thunderbolt 3. Type A and Type C connectors.
Ethernet: 10Gbps, at least optional. Gigabit has been standard for what 20 years now? Fiber optic option
WiFi: 802.11ax, rear antenna leads if someone needs a better signal than the internal antenna provide.
Display: I like the 16:10 although I'd rather 22", 28" and 34". Perhaps a 44" 8K model. Matte finish. Of course thinner bezels. I'd also want 4K to be an optional downgrade on larger sizes for those who just want a bigger screen without paying exorbitant 5K, 6K and 8K prices.
CPU: Ryzen, Threadripper in the iMac Pro. Since Intel seems stuck at the moment.
RAM: Easy upgrades by removing back, DDR4 3600Mhz, 4 slots for all regular iMac, 8 slots for iMac Pro.
Webcam: Companies put 4K cameras in $200 phones. It's time for 4K in the iMac. At least full HD. I wouldn't mind something which can do FaceID and sacrifice 4K for whatever the iPhone 11 uses for the front camera. Unless it does 4K.
Keyboard: Mechanical. Bring back the Apple Extended Keyboard II to the same exceptional standards. Use a rechargeable battery for a wireless keyboard. But use AAA NiMH so you can just swap them. Same with the mouse. Not Bluetooth something more responsive like Logitech Unify for the wireless keyboard and mouse.
Mouse: Don't bring back the Apple ADB mouse. I'd like a nice, ergonomic laser mouse with multiple buttons and a scroll wheel. Like something from Logitech. Perhaps offer left handed, right handed and neutral option. Neutral being standard.
PSU: 1,000 watt, 80+ Platinum, Made by Seasonic or Superflower. 1,200 watt in iMac Pro to accommodate Threadripper
Audio: Asus Xonar with properly isolated circuitry with rear gold plated RCA jacks and 7.1 channel SPDIF.
Cooling: Just larger as the case will be thicker to handle a desktop GPU anyways. Wouldn't want liquid because that fails.