Burdock was traditionally used to treat digestive disorders and rheumatism, the root is a blood-purifier which cleans toxins that cause skin disorders and rheumatism. It's also been used to treat cystitis and urinary stones. From what I've read it has some anti-carcinogenic properties.
Oregon Grape/Mountain Grape (Mahonia aquifolium) is one which you might be interested to try. The root was used by native Americans as a blood tonic. It's useful for treating skin disorders such as eczema, acne, psoriasis and cold sores. It also acts as a digestive and liver tonic and was used to improve appetite, suppress nausea and reduce rheumatic inflammation.
Evening Primrose (Oenotheria biennis); The blackfoot tribe used this one, they boiled the the leaf and stem and dried the roots to use in winter. The seeds contain fatty acids such as gamma-linoleic acid which reduces PMS, lowers blood pressure and is said to restore red blood mobility in sufferers of MS. Some research has shown potential benefits for hyperactivity, schizophrenia, arthritis and Parkinson's disease.
Dandelion is another useful one, the leaves have diuretic properties useful for treating urinary disorders and fluid retention without depleting body potassium. They detoxify the blood so are used for acne and eczema. The white sap is used for warts, corns and verrucas. The root is an anti-inflammatory used as a liver stimulant for jaundice, gall stones and rheumatism.
Silverweed - The native Americans and Celts would cook the root stock as food. The flowering tops are antiseptic and astringent, they can be taken as a tea for gastritis, catarrh, sort throats and diarrhea. Steeped in water and used as a wash they reduce skin redness, freckles and sunburn. If you apply the fresh plant to sore areas it relieves pain and was used as a wash to treat saddle sores on horses.
I'm sure I could find a whole bunch of others if I dug through my herbal books, let me know if you want some more ideas and I'll compile a more comprehensive list.
Regards,
David