I live in America but they don't have one where I live. So of course, I was really, really excited when I got to go to one!

I'd suggest selling a few different types of sliders, sizes of burgers, and soups. And then, of course, include "unique" milkshakes, whatever that means for you. They have this maple sort of milkshake at a place nearby, though I don't know if that'd sell well where you live. There was also this huge selection/counter of ice cream flavors, complete with sprinkles and some other toppings.

For decor, I'd go with quaint and whimsical. I forget if there are a lot of pictures on here, but this is a cute, sweet place.

*edit: I take that back; I actually think we do have one near where I live. Except it's gotten a bit too McDonald's-like for me to really go to it anymore except for reminiscing. There was a small playground outside the diner, and my parents used to take me there for ice cream and playing.