Mongosync would benefit from being able to issue a createIndexes command that doesn't block on the index build's completion, but rather only blocks on the startIndexBuild oplog entry becoming majority-committed. This could look something like:

db.runCommand(
   {
     createIndexes: <collection>,
     indexes: [
         { ... }
     ],
     returnOnStart: <true|false>,
   }
 )