There’s no recipe, you just add a pinch or two of brown sugar to deepen the color, some worcestershire sauce to taste for the flavor bump, and then any spices desired. Spice wise, it’s usually a touch of garlic powder, onion powder, blackpepper, and that’s that. Sometimes, I’ll get frisky and see what works and what doesn’t, but truth is that most spices take over too much, so it’s just about the three core spices that do well at upping the taste of the meat.
Generally, it’ll thicken enough during cooking, but if I go heavy on the worcestershire, it can need a bit of time simmering to get to the right range. But since my household prefers it fairly lightly added, that isn’t done often.
The only other thing I’ve found that improves is mushroom powder, but that stuff tends to be hard to find locally, so it isn’t a regular thing. Tbh, none of it is regular, meatloaf can be expensive, and we tend towards a more veggie based diet overall, using meats I’m smaller amounts. It’s a special occasion meal.