This was discussed in bug 1253740. storage.sync is going to use Kinto collections to store data locally, and this could also be used for storage.local. Sharing this implementation will result in simpler code for storage in general. In addition to changing the implementation of storage.local to use Kinto, this bug also needs to include a facility that will automatically migrate any data currently stored in storage.local's json files to Kinto collections.
Does this mean that storage.local entries will be separate records? I am working on data migration for Adblock Plus and was horrified to discover that all data in storage.local is being saved as a single JSON blob - for my Adblock Plus config that's currently 26 MB, and only part of that data really needs to be loaded every time.