Favorite outdoor Potluck dish thats NOT a Salad or typical Casserole

Focaccia--simply topped and precut, or fulle loaded up like a pizza. It's sturdy enough to eat out of hand and you can transport it in a pizza box.

Most requested is Garlic Parmesan Cheese pull apart bread (and variations).
It uses frozen Rhodes dinner rolls.
Great recipes at melissasouthernstylekitchen.com
Always a huge hit!
BB💐

Stuffed or fried pies that can be eaten at room or ambient temperature.
Some are individual size, some are larger and to be cut in wedges for serving.
Like Cornish pasty (meat filling), bourekas from Israel, Turkey or North Africa (cheese or potato filling), Italian filled pies (spinach, eggs, ricotta & parmesan, by Anita Sheldon; Sicilian escarole with feta, raisins, anchovies by many, here Giada De Laurentiis)
https://www.bbc.com/food/recipes/classic_cornish_pasty_67037
https://toriavey.com/toris-kitchen/cheese-bourekas/
https://www.thespruceeats.com/potato-bourekas-parve-2121871
https://www.familycookbookproject.com/recipe/698214/easter-pie--anita-sheldons-torta-di-spinaci.html
https://www.foodnetwork.com/recipes/giada-de-laurentiis/escarole-pie-2585614

I would make sesame noodles. Doesn't need refrigeration and can be made ahead of time. The recipe from America's Test Kitchen, my favorite, makes a ton and with chicken as an optional add-in.

Here's another one! Sweet Potato & Mushroom Quinoa burgers. Serve as sliders, or make "regular" size as per the recipe and just cut in half for your serving tray. Hearty for outdoors in January.
https://food52.com/recipes/71203-sweet-potato-mushroom-quinoa-burgers

I happened to see these two recipes recently - they sound tasty.

For these cauliflower tacos, a plus is that you can bring extra toppings for people to customize the basic taco that you provide (avocado, tomato salsa, chopped onion, etc.) although that could get a little messy outdoors if you have limited table space and utensils.
https://food52.com/blog/22193-hard-shell-cauliflower-tacos

These Spicy Cauliflower Red Lentil burgers sound good but to serve as "finger food," I'd make them in smaller portions as sliders. (Or just make regular size and cut them in half for your serving tray with a long toothpick to hold the layers together.)
https://food52.com/recipes/73537-spicy-cauliflower-red-lentil-burgers

How about a Muffaletta? A hearty sandwich that has to be prepared in advance and pressed?

Stromboli would work, except it might be difficult to serve and eat unless you made small, personal size versions. That being a jumping off point, you could use bread dough and create stuffed rolls using just about anything you can imagine- well, except for lettuce. If it is all inside the roll, that makes individual serving much easier. Depending on the time it will take to get to the site, you could still cook and transport things in a crockpot as well. If you take care to wrap the pot in thick towels, it most likely would hold temperature for some time. In the days before crockpots, my grandmother told me how they would cook stews in a cast iron dutch oven, and put that in a box filled with straw to stay hot for hours. She lived on a homestead in Kansas, and they had to go some distance to attend church on Sunday. It was normal to have a potluck meal after the services, and I guess that's how the ladies managed a warm meal. According to her, they had quite the variety of roast meats and other dishes, all in cast iron pots stored in straw. Otherwise, perhaps cold fried chicken?? There are also the variations on raised pies which could work as well.

Cookbookchick's "Batsaria" is wonderful warm, cold or at room temperature:
https://food52.com/recipes/25622-batsaria-phyllo-less-spanakopita