The burger chain Max in Sweden already has an app like that, so it's easily done.
https://itunes.apple.com/se/app/max-express/id334732420?mt=8
https://play.google.com/store/apps/details?id=se.max.android.locator&hl=sv
You can pick your order, any extras, what restaurant you want to pick it up from and whether it's to-go or to eat in the restaurant. You're then given an expected waiting time that's usually accurate enough that you now when to place the order just in time for it be done as you arrive. You pay by card in the app and simply flash the order number on your screen to the cashier. I use it all the time. Feels great getting to bypass the lines and have my food ready to eat less than a minute after walking inside.
Max have been using those screens for "express orders" parallell to standard cashier stations for years too. I don't think it's affected their employment numbers that much. They still need cashiers, they've just cut down the waiting times for people like me who're prepared to order ahead.