I'm pretty sure we've talked about keys before, and I'm also pretty sure we've discarded them, because although "find the key" gameplay was a cornerstone of the era Doom is from, it would really screw up DoomRL's coffee break pacing.
I fixed it!
If a key is on a level, whether it is generated or carried, there is a chance a vault will be on it with indestructible walls and a door of the same color. But they shouldn't just be vaults. It's more fun if we make the color mean something. Blue doors could hide meds and powerups. Yellow doors contain equipment and mods. Red doors contain a whole bunch of overleveled monsters.
Even more fun!
As long as we're coloring things, let's paint some levers, too. A blue lever megaspheres you when you pull it and doesn't run out. A yellow lever dumps a bunch of ammo every where. A red lever smashes you into pulp.
So why would I want a red key?
Because the colors in these descriptions were placeholders. Remember how in old school roguelikes, you wouldn't know what a wand did until you tested it? A particular color of key is a mystery until you find something to stick it in. Good luck! :V