To start any of these variations: Heat 1/4 cup oil over low heat in a small saucepan. Add 1/2 cup or so of flour*, little by little, stirring constantly. Add more or less depending on thickness. When sauce is thick, add a cup or two of warm water, bit by bit, constantly stirring, until you reach the desired thickness. The longer you keep it on the heat, the thicker it will become. From this point you can add other ingredients to make different kinds of sauces.
Gravy: Add white pepper, black pepper, salt (plenty of salt!), sage, and a bit of dill. A bit of stronger soy sauce, some onion and/or garlic powder, pre-sauteed onion/mushrooms/garlic, or nutritional yeast also make it interesting.
Cheesy sauce: Add plenty of nutritional yeast, white pepper, garlic powder, turmeric, salt. Ground mustard seed might be good in it also, but I haven't had any around to try. This is good to dip broccoli in, would probably make really good potatoes au gratin, and maybe good mac & cheese if it doesn't give you starch overload.
Casserole topping: When I bake a casserole with beans, pasta, veggies, etc. I like to spread either hummus or a roux sauce on top to keep it moist and keep things interesting. The spices depend on what dish I'm making... I'll take whatever spices I'd normally sprinkle on top (like oregano or basil) and add them to the roux and spread it on top.
* I've been told whole wheat won't really work, but I've seen recipes that call for it. I've found it doesn't end up quite as creamy, but it still makes a good sauce depending on what you're using it on.
Anyone else have variations on roux sauce they want to share?