Feature summary:

json-editor is a nice javascript lib to create html forms from json-schema.
Due to its flexibility, it can be used to edit arbitrary content.

For a multi-slot editor, for example, a schema can be automatically generated from the content models (properties = slots)

{
  "title": <page-title>,
  "type": "object",
  "properties": {
    "main": { "type": "string", "format": "visualeditor"},
    "jsondata": { "type": "string", "format": "json"}
  }
}

which renders as (wikitext => VEForAll, json=> CodeEditor/ace)

If a slot with content-model json is provided with a particular json-schema (e. g. stored under MediaWiki:Slot-jsonschema-<slot-name>.json) json-editor would automatical render an appropriate sub-editor which can be uses also as a stand-alone version:

Example slot-schema for slot "jsondata"

{
  "title": "jsondata",
  "type": "object",
  "properties": {
    "str-property": { "type": "string"},
    "int-proptery": { "type": "integer", "format": "number"},
    "obj-array-property": { 
    	"type": "array", 
    	"format": "table",
    	"items": { "$ref": "#"}
    }
  }
}

with auto-generated slot-schema

{
  "title": <page-title>,
  "type": "object",
  "properties": {
    "main": { "type": "string", "format": "visualeditor"},
    "jsondata": { "$ref": "/wiki/MediaWiki:Slot-jsonschema-jsondata.json?action=raw" }
  }
}

renders as

Of course, this could also be extended to slots with binary data (e. g. images => render upload button or drawing area)

I've already started some work here (alpha-status!), but it would be interesting to know whether this approach is of general interest.

Use case(s):
There is a general need to visually edit slot content regardless of its content model. As far as I know, there is no concrete solution available yet.

Benefits:
The approach chosen is generic and independent of the content model. It can also be built on established standards (jsonschema) and an existing code base (json-editor). The manual creation of specific forms and validation of the input is no longer necessary.