Anchovies! If you're only using anchovies for pizza and putanesca you're missing out on so much! Pop one or two of them in your tomato sauce right around when you fry the garlic before adding the tomatoes. It won't make them taste fishy but it will give it that extra something. Smush them and put them in your glaze! In your dressing!

Also less a single ingredient and more a mindset - booze. You need to use more booze in your cooking. I don't just mean "use more wine", I also mean different types too. Sake in your teriyaki, beer in your beef stew, cider in your pie filling, brandy in your stroganoff, kirch in your (sweet) pie, use it! Istg so many recipes I see online omit the booze or call it optional and it hurts me so much.