I live in a village which has a chippy, our only fast food takeaway. It was taken over last year by some new people and now not only do they do fish and chips and pie and chips, chicken and sausage (all the previous bloke did) but also scampi, pizzas, jacket potato, chilli, one of three chicken currys (tikka masala and I can't remember the other two!) and rice. They also do fish and chips in a box as Tony Day mentions, and they are always busy!
I suppose it depends what else is in your area as to whether you diversify away from the standard chippy meals, but as has already been mentioned, cleanliness, uniforms, good food and friendliness go a long way in a business.