One of the major mechanics in the Doom level design were the red, yellow and blue key cards with corresponding doors. Seeing this in DoomRL would add to the feel of having this.
Implementing this might be a pain, as you'd might have to ensure that the keys are not spawned beyond doors, or the doors do not block access to the exit. Alternatively, keys might carry over to other levels, and while having all three keys would negate the challenge provided by the doors, they would take up precious inventory space, and may not be an option in melee, pacifist or marksman runs. To keep from just shooting or shotgunning the doors open, they might be armored like walls, requiring rockets or other high powered weaponry (with precious ammunition expenditure) to bypass without keys.
Perhaps such doors would be associated with vaults and the like, set templates rather than being placed at random doors, blocking off access to items, or even special level exits.