You are right that even with no protein the best way to have a tomato sandwich is grabbing the mato warm off the bush, slicing it, and eating it with mayo between slices of good bread. But I have upped the protein by sticking a sharp flaky slice of cheddar in there, that works.
And yes, with (turkey) bacon, lettuce, and mayo is also a great way to enjoy fresh toms.
I really like grilled cheddar cheese sandwiches with a slice of tomato in the middle. Ham or bacon and tomato grilled cheese is good, too.