I am working with pipeline data and each system has a different yet similar name. As I am merging the field maps it gets very annoying to scroll through find what I'm looking for and once I'm finished I still do not feel confident that I mapped every field.
Remember those elementary school assignments where you had to draw lines from items in one column to the releated item in another column? Where Column 1 is your ouput field map and Column 2 the fields you are mapping. That's how this GUI would work.
What about when you are merging more than 2 field maps? I think it would great if the user can define what goes in the output column. This can work similarly to how you select which layers you want to show in a legend. Then once you have your ouput set you draw lines connecting fields to the new field maps, similar to how you draw lines to connect various parts of a model in ModelBuilder.
I think it would also be helpful if you could see how many fields you have mapped to each output field, this working similar to how Symbology properties can count how many records are in each field. An expandable list might also be helpful.
For some visual clarity using bold or color to distinguish fields that have already been mapped to fields that have not been mapped would be helpful so you 1) you don't map the same field twice 2) you can see what you have left to map and 3) you can verify those fields you didn't want in your output are not included.