Depends on your guests age and tastes. Not suggesting you’d serve all these things, just ideas to pick from.
If it’s more adults I’d go more accompaniments for the bbq food than different types of food so something like French sticks, garlic bread, pitta bread, pasta (or rice/couscous) salad, potato salad, tzatziki, hummus, chopped salad with feta, boiled eggs, tomato & red onion & mozarella salad, coleslaw. Slices of halloumi, veg kebabs or corn on the cob on the bbq.
If there are younger ones, veg sticks, oven chips, scotch eggs, nachos/crisps/pretzels, pasta mayo salad, quiche, garlic bread. In my experience they eat tons of hotdogs and burgers though.
for something sweet a big tub of Neapolitan with cones, sauces & sprinkles for diy ice creams, marshmallows when the bbq is nearly dead, cupcakes, fruit kebabs (melon, strawberry, grape, tin pineapple stacked on sticks) or as a salad. You can put whole bananas in skin on a bbq, after 10 mins cut open lengthways and wedge in cut up mars/snickers/bounty for another few mins and serve with ice cream.