Since the Doomguard is now gone, I believe it would be just to give it the same treatment as the old superior demons (Voidlord, Wrathguard, etc.) and come back as the glyph, “Glyph of the Doomguard”.
For this to function correctly, just replace the Demonic Tyrant’s ability “Demonfire” with the Doomguard’s respective primary casting spell “Doom Bolt”. Obviously as well as replacing the Tyrant’s model with that of the Doomguard’s.
I feel like this would satisfy a decent amount of players, and its very implementation, provide more visual customization options for Warlocks, and benefit Scribes as well.