Well... Have you tried writing an operating system yet? That is the only real overarching project for "operating system design". You could of course implement filesystems, shells, compilers, kernels (monolithic or otherwise), bootloaders etc. Essentially, you can pick a part of an operating system, and implement that.
(I'm sure you're aware that compilers and shells etc. are not part of an 'operating system' in the normal sense of the word, but they are still a low-level bit of software that is challenging to implement properly)
Last edited by Snark1994; 11-06-2012 at 06:25 PM.