I like the popcorn idea! I feel like we end up at a lot of potlucks - I usually use them as an opportunity to cook something my household won't eat, but I've definitely experienced the last minute scramble. Obviously what works will depend on what you have in the house at the time. Some things I've done:
If you have access to power at the potluck and a crock pot:
1) Hot dogs and buns - put them hot into the crock pot, bring buns on the side, bonus if you have mustard and ketchup to bring along
2) Prepared Italian beef - bring in a crock pot with buns
Room temperature:
1) Veggie tray: whatever veggies I have in the fridge that I can cut up and dip made out of sour cream and/or mayo with seasonings
2) Deviled Eggs
3) Ham / turkey rolls - lunch ham or turkey cut in strips and rolled around cheese
4) Charcuterie board - literally whatever combo of dried or fresh fruit / nuts / cheese / olives / spreads / vegetables / nice cold cuts / crackers / fancy bread I have on hand cut into small pieces and arranged together on a board or tray of some type. Somehow even if you only have plain jane raisins / peanuts / saltines (even chocolate chips!) etc. if you make them look pretty people eat them
5) Shrimp salad or cooked shrimp thawed from the freezer with cocktail sauce - I don't keep cocktail sauce on hand but I always have ketchup, horseradish and lemons and can make my own
6) Apple sauce or apple slices with sweetened cream cheese or caramel sauce (I don't keep this either, but I usually have the ingredients to make it) - Toss the apples with lemon juice or sprite to keep them from browning
7) A pan of roasted veggies (combo of potatoes / onions / cabbage / carrots / cauliflower etc ) seasoned with chili powder or even just salt and pepper - can be served room temperature

Pesto pasta salad - chunky pasta (bonus if it is tortellini) tossed with pesto, can add cherry tomatoes, pepper strips and / or cooked chicken
Dessert:
1) No bake cookies - the kind with oatmeal and peanut butter (make sure to label because peanut allergies are so common)
2) Rice Krispie treats
3) Puppy Chow (also has peanut butter so needs a label!)
4) May not work based on the particular potluck situation, but people always eat ice cream if it is served
5) If you have a can of pumpkin and time to cook it - crustless pumpkin pie
- zorkmead