I know it's been done before, but I'm pretty attached to the idea of a Halo of the Sun tattoo on my shoulder, and, if the spirit moves me, a Seal of Metatron on the other. I shall award one free internet to whomever can correctly guess what character this is a reference to. To give you an additional, and, perhaps, largely unnecessary hint, I will also spend a lot of my free time turning valves.
I know. It's possibly the nerdiest/most incredibly stupid idea of all time, but I still think I might like to get it done some day.
Alternatively, a space invader on my shoulderblade, but if I ever do end up getting a tattoo, I'd probably want it to be a koru, which is a symbol of great significance to the Maori, somehow in combination with something Celtic. I'm proud of my heritage.
I don't really know, it's something I've been thinking about more frequently as of late, because I'm seriously considering getting one soon (by which I mean in a few years or so), but haven't really decided on anything concrete as of yet.