The important principle with sandwich fillings is contrast. That means wet and dry, crunchy and soft. So, for instance, cottage cheese is nice with crisp lettuce, and dry ham with juicy tomato. Beyond that, it's largely a matter of taste.
Of course, apart from the basic British sandwich, there are lots of variations; open sandwiches, filled rolls or baguettes etc. In all cases the points about freshness and contrast still apply.