I found it easy to list my most unusual mystery ingredient, but I have too many favorites to stop there.
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Maple syrup, just enough not to taste.
It's good in anything complex, sweet-sour-hot, beefy or with sausage especially.
I like it in my eclectic chili and my mexi-talin-asian mango marinated blue corn breaded pan-fried catfish.
It's like cinnamon in spaghetti sauce, if you can recognize the taste, it's too much, but it's not nearly as unforgiving.
Other things that add mass flavor to lots of stuff:
Worcester (esp. anything with red meat)
nutritional yeast (esp. anything missing some meat or protein)
Tabasco Chipotle, or Buffalo Chipotle
(with the latter, some will swear there's tomato in there even when there's none at all)
vinegar – cider, balsamic, pomegranate, wine, rice, and/or steeped with herbs, depending on specifics
powdered sun dried wild mushrooms (can make most people swear a meatless sauce has beef in it).
oil/butter (When I was young, an old Chinese woman marveled at my stir fry, or actually it's austerity.
She said, "No oil? No sugar? No vinegar?" – much of Chinese cuisine relies on this trifecta for taste.)
Things I use whenever I can:
Mexican oregano (not the same species as oregano, which is my first lasting love with a spice)
fennel seed (I love fennel, like cumin, it ties so many cuisines together, making it easier to blend them)
garlic (sometimes the powder is better, but usually freshly minced is best)
Pearl River Bridge soy sauce
honey, molasses, sorghum, agave nectar, and/or sugar
apricots or apricot jam
yellow curry powder
red curry paste
oyster or fish sauce (both are often mostly really krill, these have a very strong "fishy" taste,
I prefer the high priced oyster sauces which are not.
Especially good in anything made with beef or seafood)
sun dried tomatoes
blue corn meal
marinade made from many of the above
The real secrets are the ones I've forgotten.
(Can't believe how many of these the spell checker says are wrong!)
Try a little red curry paste, or yellow curry powder, in your next tuna melt,
and dust the pan or bread (the butter really) with garlic powder.
It's best if the tuna has onions and celery in it. Cheddar works great.
A little extra cumin also goes well here, as might minced olives.
Cumin is in so many ethnic blends, from chili powder to garam masala,
but it's often used alone in authentic versions that otherwise use these blends,
so sometimes adding more cumin (or less) is exactly what you're really looking for.