One of the more common topping for tacos onions provide a lovely bite that helps to cut through some of the heavier proteins in tacos such as carnitas and barbacoa. They can be served thinly sliced, cubed, sauted, and pickled but are almost always appreciated for adding a bit of brightness to the flavor of a taco.
Utilized mostly for their filling nature beans (usually black or pinto) are simmered in a sauce and lightly spread on the taco. They add an earthy quality to the taco that pairs well with read meats like beef as well as white meats like chicken. Be wary of pairing with a fatty protein like pork carnitas.
Known mostly for the spice that they provide jalapenos aren't the one trick wonder that you'd think they are. Bursting with a vegetal freshness much like that of the milder green bell pepper as well a providing an almost citrus-like brightness depending on the freshness, the jalapeno provides a burst of flavor and heat to wake up the palate for a stimulating bite every single time.
Considered more of a "Tex Mex" option for a vegetable topping, the tomato nonetheless has always had a place in tacos be it in salsa or pico de gallo. So why just add plain tomato into your taco? Simple, tomatos provide a lot of juicyness and umami to the dish. They tend to be slightly acidic which helps cut through the fattiness of meats and they sort of meld together with the taco in terms of flavor.