[{id:1,name:"a"},{id:2,name:"b"},{id:4,name:"c"}]

db.collection.aggregate([{$match:{id:{$in:[1,2,3,4,5]}}},{$group:{_id:null,root:{$push:"$$ROOT"},ids:{$push:"$id"}}},{$project:{root:{$reduce:{input:[1,2,3,4,5],initialValue:"$root",in:{$cond:[{$in:["$$this","$ids"]},"$$value",{$concatArrays:["$$value",[{id:"$$this",name:"defaultvalue"}]]}]}}}}},{$unwind:"$root"},{$replaceWith:"$root"}])