I think "storing deltas" based on seed is largely how Minecraft saves the level already, but you could probably strip out a bunch of useless metadata to get it smaller

> I think "storing deltas" based on seed is largely how Minecraft saves the level already

This is not the case, level data is stored in full. One reason for this is that level generation is pretty slow (compare the time it takes to create a new world vs. load an existing one); another reason is that it changes between versions.