How is your health and what are your long term goals with RPD? Different parts of the plants, and likewise different types of plants, nurture different parts of the body just like different organs nurture different parts.
My personal choice would be long "tap"root vegetables (carrots, parsnips, daikon, beets) if I were to add a plant to my diet. The deeper the taproot the greater the minerals it will have.
Wild herbs/greens are another great addition but are obviously very seasonable and can be contaminated depending on where you gather. Dandelions in the spring are fantastic though and feel very nourishing to me.
There seems to be a lot of positive feedback here on the forum for both cucumber and celery as well so perhaps entertain these too.
Lastly, as KD recommends, a homemade ferment, such as a sauerkraut, is another excellent idea. These can be made from all sorts of vegetables too so don't limit yourself to just cabbage as a base.