I would suggest you have a few of the normal flavors a person would expect for your walk-in customers. That way, if someone walks in off the street, you're bound to have something that they'd expect you to have. Those would obviously be:
Lemon / Lemon Poppy Seed
Then, you can move into other more specialized flavors. Some that I've enjoyed include:
If you want to get really adventurous, you can also try mixing in other flavors that you might not normally find, such as:
Durian (if you're especially crazy and talented)
There are also many regional flavors that I probably haven't even heard of, so make sure to include those in the mix as well. You could try making tributes to regional flavors. For example, in the southern United States, they have biscuits and gravy as well as chicken and waffles. There are limitless options to try!
I'm no baker, so I'll leave it to the experts. What I can say, is that you should probably have a mix of expected flavors, as well as new and interesting flavors so you have a signature that no one else can copy.