When I have a tuna salad sandwich, it reminds me of the many, many sandwiches my mom made us as kids. I always loved her tuna salad. I don't there was really anything unusual about it - just your standard sandwich with very few ingredients. This is pretty close to the ones that made for us.
An 11 oz. envelope of tuna
2/3 cup mayonnaise with olive oil
1 rib celery, finely diced
1 boiled egg, chopped
Frehsly ground black pepper
I like mine on soft white bread or occasionally on toasted white or whole wheat. I've never been a big fan of tuna melts, but if you are, just throw on a slice of cheese, butter the bread on the outside and grill until the cheese melts.