Asklemmy

piss

Last year I picked a huge amount of mushrooms in the forest, dehydrated them (you can buy a dehydrator or use an oven) and ground them to a powder.
I put mushroom powder in damn near everything I cook, gives it a nice hit of umami.

Nutmeg is a criminally underutilized spice, and a little goes a long way. Damn-near everything I cook gets a little bit of nutmeg.

If what you're cooking tastes like it's missing something but you can't quite put your finger on what it is, in my experience most of the time it's acid. My go-to way to add that is with a good squirt of yellow mustard.

A little bit of cocoa powder finds its way into a lot of darker colored savory dishes like stews and such

Coffee: just put like a 16th or 32nd of a teaspoon of cayenne in the grounds, gives a depth of flavour people love. Just a miniscule amount, they should never spot it for what it is.

Balance acidity, that's pretty much how to make every sauce delicious. Per OP's suggestion, that free glutamate punch also helps.

I recently started grinding spices by hand with a mortar/pestle and salt/pepper mills, and it really made a difference. Now everything smells very nice, which really made all of the food I prepare much better. Less of a secret ingredient, but it's usually better to always have fresh spices / ingredients on hand (if possible)

LSD My cookies are wild man

hominy in chili

Butter - I use it in pizza dough, taco meat, stir fry...

I know what brands of garlic powder to use. Nothing beats fresh garlic, but a pinch of the good stuff is worth 10 shakes of supermarket brand crap.

Last mile seasoning! Shortly before your dish is ready to serve check for salt acid and heat, and adjust accordingly. This is critical!!!!

Just like adding a pinch of salt can improve any dish, adding a dash of Worcestershire sauce can improve them for the same reasons but different taste group.

And more generally, if you taste something and feel like it's missing something, go through each of the taste groups and consider if that is what it's missing. Sweet, salty, acidic, umami are the main ones (I've never felt like a dish is missing bitterness, but maybe that's a weakness in my cooking). Spicy isn't a flavour group but can add to a dish and/or mask a lack of balance.

Also, do this balancing act after you've added all the ingredients because they can bring their own biases to the dish.

Instead of salt I use Tony Chachere's seasoning. This is a staple in EVERY Louisiana household, so you know it's the real deal.

Here's an opposite example: When making lasagna, leave out the bechamel sauce. No more soggy mess

If the recipe calls for ground beef I'll usually swap it for ground sausage.

Adding half a bag of butterscotch chips sprinkled on the top of box brownie mix. I get tons of compliments like it's the best thing in the world (and it is arguably much better than without the butterscotch).