Although I too think it is meant to resemble the eye (or at least a very abstract or coded version of it), I have heard tell that it is in fact a Numenorean (s?) rune. That said, we know next to nothing of the Numenorean writing system, so I'd still say we could interpret this as a logogram rather than a phonogram.

Note that the rune is also etched into the meteoric component of the blade discovered by Theo. Perhaps Sauron retrieved Gurthang from Turin's tomb and corrupted it with his mark, enabling the reconstitution we see in the trailer. If it's not Gurthang, I don't know what else it could be. Perhaps Sauron's own blade?