My idea is to add a resolve key to the fields, so that when a value is not provided for that field the resolve function will fire, which then creates a default value for the field instead.

So for example I might have the following:

  fields: {
    title: {
      type: 'string',
      description: 'The title of the post',
      required: true,
    },
    url: {
      type: 'string',
      description: 'The URL of the post',
      required: false,
      resolve: (doc: any) => `/${doc._raw.flattenedPath}`,
  },
...

With this I can provide a URL value in the frontmatter of the markdown file if I am interested in providing a very specific URL for something, but otherwise it will generate the field automatically.