I'm not sure if you guys already thought up of this or considered it, or have it in the game already, but just in case, I really hope there are a few classes based around summoning units to help you and your teammates. They can be part of the specialization or mage class. or a mix of both. examples of what they can summon can be

skeletons (either magically or from a dead body), zombies (either magically or infecting a living creature or dead body), ghost (magically or ripped from a dead body)
fire, ice, and other elemental summons
mythical wild beasts for a "druid" type of class
and if the player wants to worship the devil, maybe he/she can open portals from hell and call forth imps, fiends, succubus, and demons