I've been experimenting with this dish for around 15 years. I've concluded that there really is no right or wrong way to do it, though there probably is a traditional way, as discussed above.
My personal preference is for pancetta, onions, fresh peperoncini & a bit of olive oi, in addition to tomatoes, of course.
I had an epiphany with this dish in Venice in the mid-1990s. I'd ordered a spaghetti amatriciana & it came out looking so pale, I at first thought they'd brought me the wrong order. It was indeed an amatriciana, made with fresh tomatoes which gave it a lighter & quite different orientation. The balance between the flavors of the tomato, pancetta, onion, peperoncini, pasta & romano was absolutely perfect.
This is an approach that you want to use to highlight a really good pasta. It's not easy to get the proportions right so that the flavors are well balanced. I've come close a few times. It's great to play with.