My go-to potluck dish used to be a mango & black bean salsa. I don’t have a go-to anymore, really. In the spring and summer, it depends on what looks good at the Farmer’s Market, what I have time for and what the host is serving/asks for (like app, side or dessert). Ooh, a winter one I’ve been doing a lot lately is this Feta Salsa: https://smittenkitchen.com/2008/12/feta-salsa/. Right now, watermelon chunks with feta and either mint or basil would be really good. Add a squeeze of lime. You could also make them into skewers, but throwing it all in a bowl is way easier.
I actually prefer hosting a brunch because then I have the evening to clean/chill. For brunch, I do a fruit salad, frittatas (which are so easy) and bagels from a local shop with either smoked salmon or smoked fish dip.