1. Comic Caricature.
While there are a number of common cartoon or comic caricature tattoos out there already, there are also a huge amount of untapped cartoon caricatures that are rarely if ever seen displayed as tattoos. A great unique tattoo idea would be getting a not so common comic caricature tattoo.
2. Abstract Art/Outsider Art.
There are many new outsider artists that create interesting drawings, shapes, forms, figures, and caricatures. Some outsider art tattoos are more simplified. Having a design made for you by the artist or buying one of his pieces could be an excellent unique tattoo idea.
3. Unique Tattoo books or collections.