This could be described as $ifNull expression that accepts multiple arguments (rather than current two only). First non-null expression would be used.

Original description:

It would be great if instead of making a long list of conditionals you could use any $any operator to get the first of the fields that exists.

Ex:

var docs = [
  { 'a': true },
  { 'b': true },
  { 'a': true}
  ...
]

{ '$project':
  { 'wow':
    { '$any': [ '$a', '$b', '$b.nested', ... ] }
  }
}

This would make it easy to project an _id for use by a $group stage in situations where you have varied document schema.

For example, the next stage of the pipeline could be:

{ $group: 
  { 
    _id: { wow: '$wow' },
    count: { '$sum': 1 }
  }
}