What are the main technical obstacles to running macOS on iPads in the near future?
Technically it is possible.
Apple made a Developer Transition Kit (DTK) available to developers back in June 2020 - i.e. the recently released M1 Macs were announced. This DTK runs macOS Big Sur on an A12Z CPU. This is the same CPU that the iPad Pro 2020 has.
The latest release of Big Sur still runs perfectly fine on the DTK, although a few things aren't perfect. For example the A12Z CPU does not contain the same virtualization functionality as the M1 - essentially making virtualization a no-go here. The M1 and the A12Z compared also has a lot of other differences - which are the cause of the big performance difference in running macOS on the two systems.
Besides the CPU itself, there are other things to take into account however - most importantly being drivers for the basic peripherals and bootloading - especially the signing of the operating system.
If Apple wanted to make macOS boot on iPads, those issues are very small and could relatively easily be overcome. After actually booting (i.e. getting the kernel running) - then comes the question of getting the actual graphical user interface running on the iPad. This involves having a suitable driver for the GPU - and drivers (which can be quite complex and involve a lot of high-level functionality) for things such as FaceID/TouchID, the flash drive, camera, audio (speakers/microphone), touch module, cell subsystem, etc.
If others wanted to try doing it, you would need a bootloading exploit to get the non-Apple-for-iPad-signed operating system to get accepted. Also you would need to acquire or create drivers for all those elements of the iPad.
Getting macOS to boot into a text-only command prompt on the iPad is probably feasible for outsiders within a reasonable time frame. Getting a full macOS working with support for all peripherals, Bluetooth mouse/keyboard and external Thunderbolt monitor - probably not something we'll see before long. Prediction is ofcourse always difficult, especially if it's about the future.