If you want to change it up, instead of mayo try mixing in a little olive oil (to hold the whole thing together) with dijon mustard (and salt and pepper to taste). Seriously - it's effin good.
I generally use the mayo concept myself, and quite often I'll kick it up with a little
piri piri sauce for spice - then I spread low fat plain yogurt on half of the bread and use that to hold in a big ol' pile of shredded ice berg lettuce. So you get the creamy heat from the tuna mixture and then the lettuce and yogurt cool things down. Taco Bell thinks they know how to do hot/cool combo? Not like this, they don't!