I'd say a Microsoft Surface Studio imitation, only with the hardware behind the display rather than the base.
Several signs point to this. About 2 years ago, Apple patented a new display stand that had the appearance of the current iMac version, albeit with a hinge at the intersect between the base and the vertical portion, which would allow the screen to either be positioned vertically or flush on desk.
Second, we've had the recent rumour about Apple releasing a Mac with an iPad-like design.
Third, just look at macOS Big Sur. Let's not kid ourselves - the U.I is designed for touch input. There was no other reason for Apple to take a seasoned and well-liked U.I design, and switch it up with something akin to iOS. The fact that macOS can now run on ARM and can launch iOS apps has little affect on the rest of the U.I.
Will it be part of the iMac line? I doubt it. Apple is moving from products with the 'i' prefix, and when you also take into account that the core functionality of this (prospective) device differs dramatically to an iMac, I can see it being an all new product line that will be the successor to iMac.