One thing to remember when putting together a brine, is the chemical state of the liquid. By adding a acidic liquid like citrus juices or vinegar you will make the brine acidic. This will tenderize meat but if it is too strong it can turn the meat to mush. If you do plan on using this kind of brine, reduce the brining time accordingly.
As for spices, imagine that you are going to be using a spice rub, but instead of applying the rub directly to the meat you simply add it to the brine. The brining process works better at pulling the flavors into the meat than applying a rub will.
Once you have the liquid chosen and added the cup of kosher salt per gallon, it's time to add the flavor. Any herb, spice, sweetener, fruit, vegetable will work. Some chefs make brines much the way you would a soup, by adding cut up vegetables along with whole peppercorns, garlic cloves, diced onion and whatever else works well with the meat you are using.
The only limit on brining is your imagination. Experimentation is the key so open up the refrigerator and the spice cabinet and start mixing.
