It's always helpful to know what zone you're referring to as well as other site specific qualities (sun exposure, flat/steep, soil type, etc,) in order to answer a question like this thoroughly.
Think of shrubs that produce abundant seeds (or fruit or berries). From personal experience, I've seen Nandina, Butterfly Bush, Mahonia, Holly (Ilex cornuta), Cherry Laurel easily re-seed. Although more rare, I've had Crape Myrtle and Camellia (C. japonica) pop up from seed.
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