It wouldn't be from the jailbreak community. It would need dedicated work along the lines of the Hackintosh community. They were incentivised by being able to use OS X/macOS on cheaper hardware. Outside of a bit of fun, there's no real incentivisation to put in the long hours required to get macOS on the iPad.
But let's say it did happen. Developers do enjoy a challenge.
I'm unsure of the nature of architectural differences between macOS and iPad. My guess would be the following probably quite difficult areas:
- Jailbreak: It'd need one of these, obviously. One that persisted across reboots if this is to be a practical realistic setup for most users.
- Boot loader: One of these would need to be created from scratch so you could choose between iPadOS and macOS. This might require hacking the firmware, which is a whole new level of pain for developers.
- Driver support: I don't know if iPad uses commodity hardware like the Mac (e.g. wifi/bluetooth adapter, sound module). If it does then there might be drivers that can be hacked. If not, drivers will need to be created from scratch. That's painful too.
And then there will be issues like getting an on-screen keyboard working under macOS. There is one built-in, if I recall, but it's not designed for touch input.