Thought I'd add in my two cents...
Jubilex (Demon Lord of Oozes)
Yeenoghu (Demon Lord of Gnolls)
Gao (Earth Genie)
Marid (Water Genie)
Dragonborn / Reptus heroes
While I can probably paint the water and earth genies a different palette (depending on the sculpt), I'd really like to see those elements represented in the sculpt to make them distinct. Everything else seems pretty obvious to me, but let me know if someone wants more details / ideas!