My fast food joint would be the “International Finger Food Emporium” – featuring a revolving menu of tasty bite-sized snacks from the world over.
ETA
A waffle joint. Nothing but waffles. Made from scratch, none of this waffle shaped pankake box mix crap in my kitchen. You can get eggs too, over hard, over easy or over medium. You can drink coffee (one flavor, no french roast, or frangelica or weasel butt or elephant dung or zimbabwean roast) OJ, milk or hot chocolate or water. There will be strawberry, blueberry or 100% real maple syrup and a few flavored diabetic syrups. there ya go, thats the entire menu right there.
They’d also go down well in Utica.
Dude, loose-meat sandwiches are comfort food for me. Many many decades ago there was actually a Maid-Rite at the end of my block. I could ride my bike to it, as opposed to the original McDonalds, which was on the other side of town.
When I lived in Cambridge, England, in the '70s, there was a Canadian hippie-type who opened a waffle house off Newmarket Road. He sold waffles with sweet (syrups, fruit, whipped cream…) and savory (ham, bacon, cheese sauce…) toppings. It was a great place to eat. 
Question:
I was looking at that Maid-Rite menu and read the description of their Classic: a ground-beef loose meat sandwich.
Is it pretty much a sloppy joe? Looks darned tasty … it just seemed to be another name for a familiar item. I don’t think I’ve ever gotten a sloppy joe anywhere except home and elementary school (obligatory Billy Madison reference 
) – I’d welcome trying one in a restaurant.
Think Sloppy Joe but sauceless. Then you can condiment it however you like. They’re messy as hell if you don’t know what you’re doing. The key is unwrapping only the bite you are about to take, just like you would do if eating a Mission burrito.
My alternate business plan, in case the waffle place fails, would be fried bread dough(elephant ears to some, scones to others, fried bread dough to me since thats what Grandma always called it when she made it). Again, made from scratch, but with a variety of complimentary sweet and savory toppings and side dishes.
Meatloaf sandwiches on Sour Dough bread, fresh lettuce, onion, and sliced tomato.
Sides of fries or chips.
Would offer three meatloaf options. Straight beef, beef & pork, or beef, pork & lamb.
It would be ideal in a food truck.
Precook the meatloaf early in the morning. Slice & serve to the lunchtime customers.
Another item on my menu would be fried egg sandwiches. Eggs would be cooked to order. On White bread or Sourdough, melted cheese, with mustard or mayo.
Gatsbies and Bunny Chows
If you put it next to a dispensary you could call it Scone of Stoned.
Bucket O’ Skin. The menu is fried chicken skin and Tater Tots, and Pepsi. The diet plate comes with Diet Pepsi. Carry out only.
Never heard of a fried scone, but fried bread dough is a Native American thing, isn’t it?
I love funnel cakes* topped with cinnamon sugar and/or stewed apples. Yum! :o
*For the uninitiated: deep-fried batter, deposited in the cooking oil through a funnel.
I’ve always thought it would be cool to have a soup place, like the Soup Nazi, but without, you know, the hostility towards the customers.
In my vision, the menu would change daily based on whatever the best ingredients were available that morning. Each day’s menu would include one or two hot soups, a cold soup, and a vegan option.* And I would have those little teeny-tiny, one-use Tabasco sauce bottles like they have (or used to have) in US military MREs.
*I would even go so far as to have separate utensils for the vegan prep, because go big or go home.
You can get a Doner Kebab just about anywhere in Britain and you’re likely able to find one anywhere in the world frequented by British tourists.
Like a gyro, but better and man do I miss them.
The perfect bar hopping food. My little stands would be all over like Subway.
Corn bread, sour cream, jalapenos, shredded cheese…I was on a trip and there was a little cafe in town that advertised ‘Chili Night’ . and for $10 you could get a bowl of any of three kinds: regular, super-hot, or vegetarian, with one refill and a free soda. There were burgers and sandwiches available, too, but they were noted for their Chili Night.
Digging this one, too. I may presume that meat loaf and fried egg can be had on the same sourdough roll?
I use beef and lamb, seasoned with cumin, to make a gyros-like meat loaf. Delicious!
Another thing I’d like to see/do is a Louisiana/New Orleans fast food joint. Offer jambalaya, gumbo, shrimp etouffee, chicken piquante, po’ boys, fried catfish… However many you can do properly on a rotating daily basis. Sort of like K-Paul’s, if that’s still around.
Chef Paul Prudhomme passed away in 2015, but K-Paul’s is still open and still popular. It’s a far cry from fast food, though – definitely a white-tablecloth place.