I guess Linux will be hard to repeat, exactly because it is so successful. There's simply no similar itch left to scratch for hobbyists. There are still people writing general operating system kernels from scratch, but they are not driven by the lack of a decent free general operating system.

Linux lacks in real-time features. That could be an angle to get in a new operating systems.

There's a saying that Unix stopped serious research into operating systems. Whenever somebody writes a new system nowadays, one of the first things they port is the Unix infrastructure.