Syntax Highlighting Idea:
Similar to the Formula Tool, allow the Multi-Field Formula tool (and other similar tools) to have Syntax Highlighting to allow user to easily determine if the formula being input doesn't have any errors.
Variable Autofill Idea:
Similar to the Formula Tool and how it provide a list of columns as you begin to type them, allow the Multi-Field Formula tool (and other similar tools) to have the ability to autofill variables such as [_CurrentField_] or [_CurentFiledName_] as you begin to type them.