Designing and implementing a custom operating system is discussed very thoroughly over YouTube. You'll likely be using fat.c with some assembly for the boot loader, registers and more. I would personally be more focused on the OS design, and basic hardware support before consulting a OS-dependent language, which I personally feel would be a logical choice in the full implementation of XYZ OS.
Windows core code never left C/ASM, so I would personally maintain these language fundamentals in the overall design.
Finally, it has already been done. This is called "TempleOS". 100% unique and not a derivative of any UNIX flavor.
Interesting story on this. HolyC (or C+) became the language the OS was written in, as well as the applications within the OS.
This may point the direction of what can be done. This one guy did it all by himself, period. It can be done. He had effective schizophrenia disorder, which just blows my mind. He is no longer with us.
"TempleOS is a 64-bit, non-preemptive multi-tasking, multi-cored, public domain, open source, ring-0-only, single address space, non-networked, PC operating system for recreational programming."