Have you thought of this below or something similar?

Here is a good one

This is The Invisible Pink Unicorn, a lesser-known symbol of skepticism and critical thinking. I have a few t-shirts that bear this symbol, in various colors (including two of my Agnostic.com profile pictures).

Leonardo's Vitruvian Man

You might consider this:
9:3:3:1
It is the phenotypic ratio that Gregor Mendel observed in the F2 generation of his dihybrid cross experiments. It is significant in that it reveals that sexual reproduction produces diversity. And this diversity, which is the raw material upon which natural selection acts, has been the driver of our evolution. And it is all that stands between any sexual species and extinction. Sex, diversity, evolution, resilience; pretty evocative for just four little digits, no?

I'd get a tattoo done with God illustrated in a very ornate composition and have it done in invisible ink. Then I could proudly boast of how beautiful it is. And how much it has added to my self-esteem being exposed for all to see. Of course there would be some infidels claiming that it isn't there and I'm only imagining it but they would include a lot of believers in gods having their first experience of what its like to be atheist.

Calvin peeing on a cross

Old school lithographs from 19th century paleontology text books make great tats, especially Trilobites and Ammonites. Subtle pro-evolution statement, too. Or a series of skulls, going from Australopithecus to us.

No tattoos for me. Can't advise you.

Perhaps a yin-yang symbol?

I think this neural network tattoo is interesting. You could sneak in a humanist or naturalism symbol somewhere in the mix, without being too blatant.

You could write “Dont know why I got this stupid tattoo because I no longer believe or am interested in what it says and now look like a right dickhead”. That might work!

Bar code

The classic Atheist symbol

Just get a tattoo of Aron Ra.

Have to ask me tomorrow. Today all I can think of is the Avengers logo.

No fucking skulllllllsssss!!!!

Or perhaps consider a more geometric design without any specific symbols?

[pinterest.com]

I just had 666 tattooed in my left ear to watch peoples reactions. All they need is to see the numbers and then i have so much information about how they think in an instant. Works great. Body language says so much.

Read the history of Prometheus, and the play Prometheus Bound from Aeschylus. I finished it desiring a tattoo of it.
Basically stealing the fire and giving to humans, he made humans aware and "independent" of the gods. His only miscalculation was that Zeus was a tyrant and would do everything to keep its authority (and he has power enough to do it).

I always wanted also the pulsars that indicate our place in the universe
[pbs.org]

Getting a tattoo? Why because it is fashonable today? OK,
I can see the need. How about a sheep?

Research the ying yang symbol and see if that resonates with you.

I have an Atheist ambigram. And I love it!

Pick a symbol of importance to you. Maybe something emphasizing humanity's place in the universe. I ran into a cyclist with the informational diagram from Voyager on her bicep. Very cool. I want the "participatory universe" symbol in ink. Diagram of a synapse, maybe? A fractal set? Your imagination is the only limit.

I don't have any or anticipate getting any, but get one if you want to. But you can't go wrong with using a symbol of comic book mythology.