The heart of the problem is that it's difficult to send specific messages through time. Languages, cultures, and symbols all evolve. However, some things are inherent. Humans have skulls and will probably have them as long as there are humans. A skull or severed head will unnerve even great apes. It makes a very concrete warning of danger. Crossbones are good not just because they are bones but because the cross shape itself forms a barrier. Red is a danger color because mammalian blood is red, and that's not going to change. Red can mean other things, but danger is always going to be on that list.
To send a message through a perilous path, it is best to repeat it. This is also like divination -- one looks for repeating messages through different symbols. So one way to communicate through deep time is to say the same thing in a lot of different ways. That increases the chance of an accurate read. The Rosetta Stone is helpful precisely because it includes the same text in several languages, providing a key from each to the others.
The really depressing part is the message itself. Humans have been sending messages through deep time since the first hominid child dragged a finger through the mud of a cave wall. (Some of those finger-flutings are still there.) We painted on walls, pecked lines in rocks, dug earth, moved stones. But we've talked about a lot of the same things. "I am here." "This place is holy." "This place is dangerous." "These plants/animals live here." "This is the burial place of a great person." What does our civilization have to say to the future? "Don't step in our dangerous garbage." We have literally made trash that can kill people for longer than our civilization will last. Other cultures left behind pyramids, chalk horses, hanging stones, statues, earthen snakes. Myths and legends. We're leaving ... garbage. >_<
It just makes me want to bake poetry onto clay tablets and throw them in a landfill.