Okay , you need to think about something first.
The fee should be a lil less as compared to the online fee when you sell the bitcoins to someone because that is what will make people use it more.
At the same time there have been problems with the ATM when the person transferred the money but had to wait a lot lot more than the normal waiting time therefore you do need to thinking about solving such kinds of problems instantly.
It is rather a good idea but you need to improve the existing idea of Bitcoin's ATM .