Just remember to use the low setting on yoru slow cooker to avoid drying out the ribs. Any combination of the following will make an excellent dry rub for you:
granulated onion powder, or chopped onion
granulated garlic powder, or chopped garlic
Black or white ground pepper
The trick is to ballance the flavors. Typically, most dry rubs use some or all of the above listed spices. You can also add herbs such as thyme, sage, cumin, coriander, even spices such as closves and nutmeg will work. Mix together a few of the ingredients and then wet your finger tip. Dip it into the spice mixture and pop it into your mouth. See what it all tastes like together. Then, modify as needed. Sweet things go great with pork, hence the brown sugar. A little heat from the chili pepper and ground pepper, and some savory ingredients all ballance each other to give the final flavor profile. With rubs, let the meat sit in a sealed plastic bag, in the fridge for several hours to let the flavors permeate the meat, before cooking.
Have fun experimenting with different flavors. And remember, you can also marinate in fruit juices or spread fruit puree onto the meat before cooking it to give even different flavors. Apple, bannana, orange/citrus fruits, cherries, mango, pineapple, and many others compliment pork wonderfully. Even maple syrup can be slathered over the pork ribs before they are slow cooked to perfection. Add a bit of liquid smoke to some good grade-b maple syrup, and bake in a slow oven for several hours in a covered roasting pan to get fall-off-the-bone ribs with a smoky maple glaze.
I hope that gets your imaginative juices rolling.
Seeeeeeya; Goodweed of the North