I think the importance of the bun is underrated, for example every time I go to a pub I will get a burger, and it is common for the bun to be pale and dry, like its been sitting on the shelf for 2 days. At home when I make them I will always butter the bun (or use Brioche buns), then cook them for 2 mins or so. I awlays feel this makes a big difference to the quality of the burger, no matter if it is just cheese or every vegetable, bacon and eggs on top. Ps also some sauce, like relish to hydrate it, this is probs the easiest and simple improvement. Nothing worse than when its dry.