I want something that, when you look at it, you think "oh, that's marty".
Something that my secret army can spraypaint bad approximations of on brick walls. Something the future insurgents can vandalize and vilify. Something for future governments to ban the display of.
What sort of imagery is appropriate? I don't know. Mars stuff. Alien stuff. Midgard stuff. Yggdrasil.. whatever. I don't know.
Not a big 'M'. No typography at all, I'm thinking.
Trying to find examples of historical seals on the web has proven difficult. Here's a site that sells reproductions tho, and has a decent selection.