Many items make for good bitter notes. Gentian is the base for most commercial aromatic bitters. Hops would obviously add tremendous variety to the mix. Citrusy hops would probably lose some of their "statement" in bitters... I have not boiled hops, just use alcohol tinctures.
I add simple syrup, honey, or molasses to sweeten.
Different spices and herbs add nice touches depending on the intended use.
Clove is very strong and easy to overdo.
Cardamom is very good, but use whole pods.
Licorice root, seed, and bark are all good but have a documented ability to raise blood pressure.
Aromatic bitters usually do not include much citrus. For citrus bitters I use lots of different citrus. Ordinary navel orange peel is a real disappointment. Clemantines, blood and bitter orange peels are very good, as is grapefruit peel. I chop up the peel and dry it first. If just soaked while wet it seems to dilute the flavor. Most citrus bitters are around 15%. Lime peel soaked in tequila is the base of a Margarita bitter I real like.
Cinnamon, nutmeg, star anise, fennel, dill seed, caraway all have distinct flavors.
Elder flowers, lavendar, dried roses add more subtle aromatic tones.
Look to brewing supplies and health food stores for the best ingredients and ideas.
Right now I have wormwood, mugwort, sassparilla, and more new stuff working.
