To move past traditional depictions of the lotus and really humanize it, consider the paintings of Georgia O'Keeffe, whose erotic vision of flowers was in line with the Hindu concept of the lotus as the "Divine Vulva." The lotus would make a great navel tattoo. Hindus depict it springing from the navel of Lord Vishnu; through it, he gave birth to the Brahma.
Circular designs make for strong tattoo images, and the dharma wheel can be as elaborate or simple as your vision and budget allow. It symbolizes the teachings of the Buddha. His followers imagined it rolling through the world and spreading change. Combine it with other transformational images--for example, the butterfly.
While sitting under this tree, Siddhartha Guatama attained enlightenment and became the Buddha. Add an apple or a snake, or both, to juxtapose this Buddhist symbol with the Judeo-Christian symbol of the Tree of Knowledge of Good and Evil.
Snakes and serpents are popular tattoo images. The Naga, in Buddhist tradition, is represented as a cobra and is seen as a protective deity, rather than a source of evil. In some representations, the Naga has more than one head. Combine the evil Western serpent with the spiritual Naga for a yin-yang effect.
Traditionally he is a chubby Asian man sitting in the lotus position. To express the universality of his teachings, you can change his race or even his gender. The Buddha is often shown surrounded by many other Buddhist symbols, but to further illuminate his universality, combine his image with symbols from different religions.