If / when you try this, let me know how it turns out!
Also, I’ve heard that the longer you cook the roux, the weaker its thickening properties becomes. So a chocolate-brown roux cooked for, say, a gumbo will not have the thickening power of a blonde roux. Just something for us home cooks to keep in mind.
Beef stock doesn’t seem right, at all.
I will! It doesn’t repulse me, just seems an odd choice. Maybe I’ll try it for my private Thanksgiving dinner. 
I hadn’t learned that, so thanks for sharing. It makes sense. I almost never cook a roux to chocolate brown, usually just tawny, and that works well for my gravy-making purposes. I tend to rely on the fond left in the pan to make darker colored gravies.
Agree on all counts. As for the first sentence, I was following my mother’s somewhat vague instructions. When she said “add an equal amount of flour to the drippings”, I didn’t realize that she didn’t mean ALL of the drippings, so I probably added over a cup of flour. She also didn’t mention browning the flour. Anyway, when I started adding water (no mention of stock), it quickly turned into several quarts of a dubiously pale glue. Everyone politely ate some. When I went back to the kitchen and tried to take the ladle out of the gravy, I ended up lifting the whole pan up. So yeah. . .concrete. In my defense, I had never cooked a turkey or made gravy in my life up to that point (I was 21).
That’s a pretty funny story to tell later, even though I’m sure it was upsetting at the time.
I bake my bacon and save the grease (aka bacon nectar) in a jar in the fridge. (best before for refrigerated grease is infinity) 
You can do the same with your beef drippings (when you make ground beef or a roast and aren’t making beef gravy) - the beef jus and fat will separate. Accumulate that in a jar by itself and now you have some beef tallow to use in such circumstances. Beef tallow has a more pleasant flavour than bacon grease if you’re cooking something that you want to keep the flavour profile more mild - such as chicken gravy.
Wouldn’t it be better to use chicken fat for chicken gravy? I like my gravy to reinforce the flavor of the main course.
You could also save that in your fridge….to ensure it has a long shelf life, you need to separate the fat from the jus. The jus will go bad (likely after a week or so), so you need to separate the two - once you have the fat in a jar by itself, it should keep virtually forever when refrigerated. It, like bacon grease or beef tallow - should be solid and light in colour. Lard is actually pig fat - and may also be a good substitute when you don’t have enough chicken fat available.
I tend to cook a lot more beef than chicken - so easier to accumulate the beef tallow.
I guess i cook meat a lot more often than i make gravy. And i pretty much only make gravy with a roast, so the pan drippings contain plenty of fat. But i was once served pork gravy with chicken, at a cafeteria, and it was really weird. It looked like it could be chicken gravy… But it tasted of pork. It just felt really wrong.
My mom used to make pork gravy and serve it over rice while I was growing up (1970s). It was so good.
I only make gravy when I roast a chicken: pour off the drippings, scrape the goodness off the roasting pan, and tilt the bird so any residual liquid joins in. That, water, butter, and flour are all I use. I like it pretty thick, so I use lots of roux. Because my wife hates all gravy, my kids and I can’t eat it all, and I toss whatever’s left over into the chicken stock that I make out of the carcass.
Absolutely. From experience, using a white roux as a baseline, I would say a peanut butter-colored roux has about 2/3 - 3/4 the thickening power, and a medium brown, like milk chocolate roux, is getting at about 1/2 the thickening power. If you get into the seriously dark Cajun rouxes, it may be closer to 1/4 thickening power. This is just ballparking it by my experience. I don’t know if there’s any actual charts on this or not.
As for gravies, I use whatever fat I have available. I don’t have any issues with butter, oil, animal fats. They all work fine. I keep a jar of mixed animal fats (bacon grease, chicken fat, beef fat, pork fat) from whatever proteins I’ve been cooking over time, and use that as a general cooking fat in recipes. I don’t necessarily care about matching chicken fat to chicken broth or anything like that. Sometimes I do. Sometimes I want a little contrast or additional flavor. There’s no real rules to this. (Like I love making beef stews and bolognese with chicken broth rather than beef.)
That’s what the filé and okra are for.
Yep. Cajun roux can be a burnt mess in an instant. This is a skillet you never look away from till after you’ve added liquid.
My hack to fix a gravy that is too light, add some soy sauce at the end. Before correcting the salt.
I specifically use dark soy sauce for that. It’s for caramel color in it and a little goes a long way in coloring your sauces. It works much much better at darkening than regular (light) soy sauce and you don’t need anywhere near as much of it so it doesn’t affect the flavor too much.