Well Death Knights are kinda-sorta in BBI, though there are only a few of them.
The Living Armors? Simple, have them as a family variant that trades the shield for a lantern with disabling and incapacitating magic and more aggressive swordplay. I don't know if they're going to adhere to some clause in the lore that keeps Death bound to BBI, but mini-bosses that carry on his likeness are an acceptable substitute.
Ghosts are basically in already, not sure how you'd distinguish them from the wisp type enemies.
As Spectral-type enemies that
don't rely on slow movement and possession to deal and be susceptible to damage, maybe as an inverse that flies around quickly and deals pure contact damage. Ghosts work best in flying swarms after all.
Medusae could be interesting, though stoning probably would be a temporary debuff and health drain of some sort instead of an instakill.
I'd rather their take on petrification be through a channeled gaze attack rather than black mist the cockatrices spend so much time charging up. Honestly I'd prefer it if petrification were altered to increase purely on exposure time rather than gradually following exposure, I think it could lead to funny outcomes with parties of stiff, nigh-statues having to gimpily drag themselves around before being cured entirely.
Manticores would be badass and they could probably make the base pretty easily by modifying the chimera model and moveset.
Trade the goat and the snake tail for a poison stinger and attacks that make use of it.
Why do you want to see sphinxes? What would you do with them inside a DD style game, assuming they don't bring in dialogue?
Ideally I'd see them as a kind apex-predator, boss figurehead of the harpy family. Have their cries be able to charm pawns and give them access to lightning and holy magic.
But there is another
reason.
See those tits? You can climb them.
Trolls might be interesting if they have a regeneration and regeneration stopping mechanic.
That, and I think they have a niche they can occupy. I don't know if they were anywhere else in the game, but I remember BBI having smaller, adolescent cyclopses that popped up from time to time. Something half the height, but faster and more aggressive.
Orcs are just sort of boring and in my mind, basically covered by the larger hobgoblin enemies.
Truly? I always saw the hobgoblins as stand-ins for proper Orcs. Ideally, I'd want Orcs to come in at the same time as a goblin expansion; aside from grimgoblins they always felt so puny and ineffectual, don't really take much advantage of numbers or mobbing as much as short group enemies should. The average Orc would be more aggressive, combat-ready and roughly equal to the average pawn.
Oh yeah, and DDO also had
ents.