Maybe it would help to know what the campaign has already
established thus far.
So far, the main opponents have generally been the PC races as thugs, "evil" spellcasters, bandits, etc. And I agree that when it works it's a great option.
The "evil humanoids" they've encountered thus far in the campaign include...
*Swamp Hag (modified sea hag with sea witch levels)
*dark stalkers, dark creepers from FF (I presented these as unique creations of one particular swamp hag)
*boggles from MMII (mutated and regular) -- again not a widespread or well-known race by any means
So in other words, I have not established that there is a widespread "evil humanoid" race as of yet.
The undead they have encountered so far:
*incorporeal and corporeal undead per the templates in the AU book, using warmains, witches, magisters, and my "akashic monk" variant as the base creature, usually human.
*shadows
Nothing else really (no standard skeletons, zombies, ghouls, etc.).
The "fiends" they have encountered so far include:
*vultrek (from one of the published AU adventures) (unique thus far), *half-fiend monstrous centipedes (unique)
The existing AU and D&D creatures I'm definitely not going to use include:
*goblins, orcs, ogres, etc. Kobolds only get in because of the mojh. Too "D&D" for AU in my opinion.
*standard trolls (I do like the medium-sized Forest Troll from MMIII, I might make their claws poisonous instead of their weapons)
*rhodin, chorim, alabast, etc. (just not very interesting to me -- and it is here that my DT starts to diverge from the published setting, for the less I use monsters common to the setting, the more I have to find good substitutes as necessary)
One humanoid race that does interest me from AU is the harrid -- that's a good sort of "magical overlord" type race if given some spellcaster levels, but I fear my players will not be able to see the difference between them and the somewhat avian vultrek they've already encountered.
So the roles I'm looking to fill, and my thoughts on each:
various kinds of evil humanoids (fills the D&D orc/goblin role):
*ibixian (goat-folk) with tweaks necessary to make them any more interesting than rhodin; a good orc or hobgoblin replacement (MMIII)
*poison dusk lizardfolk (a good kobold replacement) (MMIII)
*kenku (MMIII) -- a sneaky humanoid, not sure if it's a direct replacement but maybe a good goblin replacement in some situations (MMIII)
*grickling (a humanoid/bipedal version of the grick from MM -- it's got slashing tentacles coming out of its face

)
*forest troll from MMIII
various kinds of big evil humanoids (replaces troll and ogre and generally evil giants):
*hook horror
*earth hulk (a humanoid/bipedal version of the bulette with the earth subtype)
various kinds of neutral humanoids (fills the lizardfolk role in a sense):
*akthar from Ruins of Intrigue (liking them better and better)
*azers (I have established that there's at least one colony of them under the mountains; perhaps they'd make a good dwarf substitute in some situations)
*skulk from FF?
various kinds of civilized evil humanoids (fills the drow role):
*take a closer look at the valorians from Legacy of the Dragons
*look at some of the psionic races from ExPsiHB -- maybe the maenad for instance, and maybe spruce them up for AU
mastermind-type monsters (fills the beholder and mind flayer role):
*beefed-up or leveled-up grells (Lords of Madness) (maybe use half-ilithid template from FF on them) -- actually with their floating, they do make a good beholder substitute, and their brain-like appearance and tentacles make a good mind flayer substitute.
*use the replacement monsters created by Ari Marmell in his Iconic Beastiary (phrenic scourge = mind flayer; evil eye = beholder) -- they are twisted enough that they are not immediately recognizable as replacements
*various kinds of demons and devils from all sorts of products -- the weirder the better -- would work in these roles as well.
*nagas with levels
The underdark Races (some already mentioned above)
drow = ? still haven't figured this one out, but maybe valorians, or maybe something simpler like fiendish humans? or what about nagas or yuan-ti or something? Hmmm....
kuo-toa = gricklings?
svirfneblin = azers?
goblinoids = athkar, goatfolk
keep the derro 'cause they're cool

mind flayers/beholders = grell philosophers and the occasional other oddity as mentioned above
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I know I am having trouble explaining myself; but it is very helpful to just hear suggestions as they jog the old brain in new directions. Thanks to all who have tried to help out.