Well here are some suggestions:
The Smith could be based of the S1 Hell Golem model (http://www.sacredwiki.org/images/Enemies/monsters/hellgolemgreypic.jpg)
Fangskin shares some similarities with S1's wreathed chokers (http://www.sacredwiki.org/images/Enemies/monsters/WreathedChokerMOnsters.jpg)
Battlemaid Sarina could either be an Inquisition Guard or you could try to implement the S1 model of Dark Elf Matriarchs (http://www.sacredwiki.org/images/Enemies/elves/darkelvenpriestesselves.jpg)
Icehawk riftwing could either be a S1 Golem or even a Sakkara Demon
The members of the travincal high council (ismail and company) could be made of Sacred 2 high elf inquisitors (or human dark mages).
Izual is supposedly a fallen angel, so I guess that either the Dark Seraphim or the common Seraphim with a black and white version of sophia's armor could fit. Yeah I know he's a huge male demon-boss in the game, but nobody minds boobs once in a while.
Diablo (actually, Diablo in D3), bears some resemblance with anducar's demon form, might be worth a try?
The three barbarian super uniques in D2 LOD could be replaced with shadow warrior npcs using a pretty set of armor and weapons.
Some other models which have been rarely used in game could also work out as bosses/superuniques, as they are either useless NPCs or very rarely seen: Witches , Undead Legion Mages, S1's Anducar and Shaddar, etc.
Another funny idea for superuniques would be actual characters (like those in the Age of Heroes mod) using a certain set of skills, emulating pvp. Considering the new mounts should also work if used for NPCs, I belive battling a high elf boss riding her dragon pet would be quite amazing.