Something that already exists on the PC. You can trivially boot up any operating system you want on any PC and the basic things like the display and the input devices will just work.
This is because the PC platform not only has certain basic hardware components standardised, but also because there are interfaces to enumerate the hardware you have. Once you have your kernel in memory and running, it can simply look for the hardware and access the hardware accordingly.
On ARM there is no such thing as a PCI bus. Therefore your kernel needs to be compiled for the very device you want to use it for. You cannot just compile in the most common ethernet controllers into your kernel and expect it to choose the right one. This may work, but very likely your first driver will try to probe blindly for its device, crashing your system, before the second one even has a chance to run.
This is why there are movements to create a common hardware interface, one where you just have a single operating system image running on a huge variation of hardware just like on the PC. Unfortunately the business model of ARM doesn't help here. ARM licenses its cores to many SoC manufacturers. Each one of them hopes to lock in its customers making it deliberately hard to switch to any of their competitors. A common interface would sweep away the borders. You could switch from manufacturer A to manufacturer B just like you can switch from a Dell PC to an HP one.