Say you wanted to express the following SQL in MQL, it would be difficult to do today:

CASE
     WHEN foo = "bar"
     THEN (SELECT x, y FROM tbl1)
     ELSE (SELECT x, y WHERE x > 10 FROM tbl2)
END

You could hack around it with $facet or $function, but it would be better to provide a first-class MQL operator to do this, e.g. an aggregation stage version of $switch (we already have a $switch at the expression level, but...)

{$switch: {
   branches: [
      {case: {$eq: ["$foo", "bar"]}, then: [], collection: "tbl1"},
   ]
   default: {
      pipeline: [{$match: {x > 0}}],
      collection: "tbl2"
   }
}}