An "open data store" as a standardized service could be interesting... a good chunk of backends are just "add this item", "get all items since X", etc.

If there was a service that gave you an endpoint that you could distribute to other services to do just that (one endpoint per service, keep things isolated), or alternatively host your own and give that endpoint to services... it'd be cool. The tricky part is getting other companies to agree to use your common API.