Each addon should just save its own data. Every addon providing this cooperative function would otherwise just overwrite each other on loading, and possibly provide different data, since you can't just hook and call the prev overwritten function for more data without doing even more processing.
GetMinimapShape() only worked because only ONE minimap addon can be running at any one time. Itemprice worked because there is only ONE price for an item, and if the price doesn't exist in database A, it can call the hooked functions in database B to retrieve it when A overwrote B.
In this case, the size of the data being returned is unknown. Such a standard API would only work if there is a "standard piece of code" that every author can cut and paste into their addon, with a small editable section where an author can then insert his own data, with hooks to every addon in the chain.