It all sounds funny but the final solution is not far away except the JSON bit. FriendFeed 11 years ago first popularized the concept of storing schema-less data in MySQL: https://news.ycombinator.com/item?id=496946. Uber did the similar thing a few years ago: https://news.ycombinator.com/item?id=16251143

I've been building an open-source alternative on mobile that based on similar concept (SQLite + FlatBuffers): https://dflat.io/ SQLite own schema is already awesome, but in this way, you can have sum-types, better schema upgrade guarantees, index building can be asynchronously etc.