When I was young we made use of anything at hand for a sandwich filling. Some sounds quite common like the last fried egg from breakfast and some not so common like mashed new england style baked beans on whole wheat bread with a slice of pickle. Scraping the dripping from the cold pork roast pan was good if mama didn't get after us.
Any leftover salad with great crunch makes a great sandwich. Chopped cheese,parmesan,sunflower seeds,egg,beans or cottage cheese makes a great protein additive.
Peanut butter and banana is a good second for favs.
Sardines and dill pickles between two slices of buttered bread. You can lay the whole sardines between the slices with thinly sliced dill pickles or mash the sardines. You can also chop up the pickles with the sardines and make a spread.
Cream equal amounts of butter and marmite into a light brown mixture. Smear on two slices of bread. Embed sliced banana (1/8" thick). Yum!
When I'm heating or cooking spaghetti sauce, pizza sauce, etc, I can't resist tearing off a hunk of French bread, adding some butter and dipping it into the bubbling sauce.
I love cashew butter on rye breads, especially savory cashew butters. Honey and a little bit of salt makes just about everything better too