I wonder whether filesystems could be more layered. You can already do this to some extent with LUKS and LVM on Linux, but could you go further? Rather than having a big monolithic filesystem like ext4, could you run some simpler filesystem that just did journaling, then on top of that one that did files and directories, and so on and so forth. By separating featurs out into layers, you could potentially improve stability a lot by getting rid of the features you don't need, instead of having to replace the whole FS with a completely different one.
That being said, I suspect a lot of the code is shared in the kernel already.
