The Cyberdemon's theme has hidden pentagrams and 666's that can be viewed in the song's spectrograph.
The trophy/achievement "Shoot it Until it Dies" earned after defeating the Cyberdemon boss is a reference to an infamous "pro tip" once created by Doomworld co-founder Andrew "Linguica" Stine as an April Fool's joke that over the years has been mistaken for a genuine scan from an old GamePro magazine.
Images leaked from the time the game was still being called Doom 4 show a completely different looking game. The setting seems to have been post-apocalyptic, and some images show soldiers in a broken down city. Not much else is known about this early version.