Linux is technically just the kernel, which is essentially device drivers. The rest of the OS that talks to the kemel and runs the UI and services is other software, most of which is from the GNU Project of Richard Stallman.
It would be a mammoth task to create another OS but there’s no legal reason why you couldn’t as long as you don’t copy verbatim from other OS’s
Also, Linux/GNU Linux is alot more than just open source. It’s core foundation is Freedom: the freedom of the human being to have the code, read it, use it, modify it and share it. As long as you give back to the original source any improvements you make.
This freedom is what sets it apart from all other OS’, even other open source one’s.