What do you think? I've always dreamed on something like this, and I hope they implement someday. I know locks already have plenty of toys and will be getting even more.
[Glyph of demonic illusion] minor glyph
Teaches you Demonic Illusion.
Alters your minion's appearance to something less threatening. The illusion breaks upon combat. 3 second cast. 30 minutes cooldown.
So, when out of combat, your demon will have another appearance and a "common" name, as follows:
Imp: Monkey, similar to the companion pet
Voidwalker: Fire elemental, as Destruction warlocks can pass for fire mages
Succubus: Random female humanoid of your race, dressed in common clothes
Felhunter: Random skin of wolf or mastiff
Felguard: Alliance gets a male Draenei. Horde gets a male Orc. Both dressing warrior-style.