I suspect that this topic has been approached in the past, but I want to bring my own 2 cents to the table (if you'll have my cents ::- D).
Since we can add onComplete and other such callbacks upon construction, why can't we add it later directly in the instance (if we saved the reference to the instance of the TimeLine or Tween).
For example, in my situation, I want to add an onComplete to a TimeLine, but only sometimes. It's clumsy to call the constructor in an if statement. I would prefer to call the constructor and append methods and only later add the onComplete event (or any other such events).
Are you just saving space in the library? Because as far as I suspect, this would be a piece of cake to make.
Could you perhaps advise how to implement this as an extension to your library? I'm thinking of extending the TimeLine class and add the properties myself. Size is not a concern for me (these days, for who is?).