I think it would be interesting to create a file system which allowed both positive and negative offsets and would behave like any normal file unless you did a negative absolute seek or native relative seeks that add to a negative value.

The idea would be to keep metadata in the inode that would note the start of the file from a seek point and it isn't necessarily zero but would map to zero. Perhaps even allow the negative portion to be encrypted separately from the normal positive section. I'm
just thinking a lot of stupid meta data add-ons could be absorbed into the main file.