Sheets does get old after a while How about a monthly special on the MTO menu. Change it up each month for something different. Hot Cuban sandwich, hot sausage, Reuben, etc.
If I want to order two of the same thing I now have to go through the full process twice. Why can't I just have the option to say I want 2 or 3 or 10 of the MTO item I just built?
I believe there should be heavy/light options as well as a "see me" button for each individual item ordered in the case of a request to swap a honey mustard sauce vs mayo swap as brought up by another poster.
Allow customers to decide if they want their sub rolls toasted or not. Not all customers want to have toasted rolls on hot subs. Also place a special request button of the touch screen order machine. That way special directions can be made to a "Made to Order" sub. If you do not want subs "Made to Order" then change the slogan to "Made the Sheetz Way, so Deal With It"
Make everything on the menu able to be completely customized. Having to yell back to the busy workers is too hard on both customer and workers and then you often end up with something on your food that you don't want/are allergic to/etc.