I'm just going by what the rumor is saying which is a 27" Mini-LED 120Hz "ProMotion" monitor. It'd have to be 5k to be a "Retina" display per Apple's definition and Apple doesn't do non-Retina displays anymore. I'm not sure if DSC would make it possible or not for the bit depth Apple would want to use, or if they plan to use two cables or what (Mac Pro?).
Maybe its not 120Hz, but variable refresh rate 60Hz, which would be better than the Apple Studio Display which doesn't have VRR, but I'm not sure they'd call it "ProMotion" in that case.
A 24" 4k Mini-LED ProMotion display is also possible.
Personally I don't see any new display from Apple any time soon after the Apple Studio Display. Maybe they have something in the works, but I imagine they are going to keep working on it. The Apple Studio Display is pretty unimpressive, especially at its price in 2022, but there aren't many 5k options, and there will be enough Mac users who buy it for 5k and the integration.
For myself, I'm looking at the upcoming crop of FALD 4k 144+ Hz monitors coming out and will either pick one of those, or look for a deal on edge-lit 4k 144Hz.