So, if I understand you are going to add...
-ArchVile (I don't know how they would work though)
-Mancubus (Again, it would be a strange addition, it realy depends on how you make them)
-Pain Elemental (we are going to beat down a load of lost soul :P)
-Revenant (kinda weird :P)
-Spider Mastermind
The only missing monster is the Spectre, but it doesn't fit imo (and how would it works anyway?)
You could always add the "boss", that big wall, it should spawn monsters continuously and you would need to activate something, then go to a specific area... this area would go up and you could shot the boss once. Then they go down again and you need to activate the things again...
Other monster I may like to see...
(from Doom 64/Absolution)
-Nightmare Imp (Ok, Im not sure anyway if they would be any different from the normal Imp anyway...)
-Nightmare Spectre (Yeah, a Demon with 3x more health :P)
-Nightmare Cacodemon (a little harder to kill, rapidly throw 3 fireball..)
-Motherdemon (for a boss, it could be nice)
IMO however, I don't think monsters that aren't in any Doom game should belong in DoomRL, and if any comes in, better give it a realy good reason :P