It would be nice to have the ability to supply variables to a transformer, just like how you supply variables to a function in a programming language. This allows to write more universal transformers that can apply in different situations.
Within the transformer: Specify a list of keys and their types (boolean, integer, string, array, object etc)
Within the place where you want to use the transformer: A list of parameter fields appear, showing the key and a text field wherein you can supply the value.