Depending on the sandwich I make a spread that works for the sandwich.
For some I do the mayo a bit of sour cream or Greek style yogurt and relish. I use that on lots of different sandwiches.
I like making a ham spread, Toss in a blender or processor ham, pickles a bit of mustard and a bit of mayo. It is sort of like a Deviled Ham spread.
For beef sandwiches I make a mayo/sour cream and horseradish spread. If it is just for me I use quite a bit of horseradish.