Not sure if this is helpful or not. On one hand, you say you want to focus on waffles and doughnuts and on the other hand, you are asking for other dessert items to add to the menu so I'll throw this out there.
I don't have much of a sweet tooth so I'd be unlikely to frequent a dessert-only spot on my own but if I was with friends who were so inclined, I'd appreciate finding some less-sweet or even savory options.
You may already be planning savory options for the waffles. Even if your focus is desserts, someone like me would appreciate the savory options, like the ones offered at this waffle place.
Have you considered crêpes? Lots of options for sweet and savory versions, as can be seen in this menu from a place near me.
Savory pastry - if you do anything based on puff pastry (either homemade or purchased frozen), you could offer little spinach, mushroom or cheese tarts in addition to sweet pastries.
Small items like cookies or biscuits that might be offered either with or without icing. If you're going to offer ice cream eventually, the cookies/biscuits can be used to make ice cream sandwiches.
Fresh fruit bowls would be a nice option as well.
