[Rockvole]

It would be simpler to have something like a google drive / dropbox where you allow access to store things on there to specific apps. Then you allow the data to be retrieved by another app. If the storing app does an export (say in csv format) each time you make changes then the data will always be available for another app to use. I think requiring an app to use a specific external database probably wont work because app developers have specific needs for each app, perhaps xml or key-value pairs etc. The app is then relying on a third party to provide the performance / latency they need. If a universal database could be chosen I would choose sqlite since that is available on phones.

[chromano]

Yeap, but storing data in things like dropbox/drive/s3 would require the app to have some sort of indexing in place (I'm thinking about ElasticSearch), as querying on these storages would perform really bad I think.