Proposed TransactionBuilder Interfase.

gistfile1.js

/* create and sign */

    var tx = TransactionBuilder.init(opts)
      .addUtxos(utxos)
      .addOutputs(outs)
      .addKeys(keys)
      .build();

   broadcast(tx.serialize());      
    
/* create and sign later */

    var builder = TransactionBuilder.init(opts)
      .addUtxos(utxos)
      .addOutputs(outs);

    var tx = builder.build();

    /*..later..*/

    var tx = builder.addKeys(keys).build();
    while ( tx.isComplete() ) {
      /*...get new keys ..*/
      tx = builder.addKeys(keys).build();
    }

   broadcast(tx.serialize());
   
/* opts for init:  fee (fixed), remainderAddress, spendUnconfirmed, locktime, signhash, */