The gamepad object is missing the expression "button index: returns the index of the last button pressed" (need this for Axis index too).
Also, the conditions "Is button down", "On button pressed", and "Compare Axis" need to be changed from a list of predefined choices to a field that allows for variable values so we can use a button index (or allow for the user to choose between either variable field or the predefined list.)
This will allow for creating custom controls.
If this is something that's unlikely to ever happen (or is not possible due to HTML5 limitations,) that would also be good to know so I can consider alternate methods.