After trying a variety of things in the past couple years, what finally works for me is just a little salt (1/2-1 tsp in to 28oz can), some oregano (2 tsp) and olive oil (1 Tbsp) in my crushed tomatoes-- and I could live with just the salt and oregano. At one time or another I've tried most of the typical ingredients (including balsamic vinegar, which seems more like adding sugar than acid-- I would choose red wine vinegar over it, and using pecorino romano in lieu of salt, which adds its own funky thing but kind of kills the brightness of the tomatoes). Never been interested in trying fennel since that means Italian sausage. I think I've accepted that perhaps unless you are making plain cheese, mostly clean and simple tomatoes is what you really want; although I do rather like onion and garlic if it doesn't get too busy, i.e.: there isn't a bunch of spices in the toppings-- processed meats and the like-- already.