Why do you think sub sandwich restaurants with a drive through have never caught on? Surely most people would be ok eating one without personally picking all the ingredients?

People won't know if you made it when you ordered it or earlier in the day. I get it but you would have to keep it simple. No Ham & cheese with green lettuce, red onions, etc. It would be chunks of ham, melted velveeta and grilled onions. BAM. Here's your sammich, shot with a sammich cannon right through the window!