Consider mashed avocado and mayo. I had sandwiches made this way in Chile. They also included a slice of tomato and called it an Italiano. I had fancy ones with grilled loin and cheap ones with chicken or hamburger. The amount of mayo was eye-popping and so was the quantity of mashed avocado. The combo was really, really outstanding. Put on three times as much mayo and mashed avocado as you consider reasonable.