One of the things I miss in Warlock are different schools of magic. They exist in other games of this kind, like Master of Magic and Age of Wonders series - different wizards specialize in different schools of magic that enable them to research different spells. There are religious spells that differentiate wizards in later stages of the game, but still I don't like the fact that all wizards can research all kinds of non-religious ones. There should be wizards that specialize in Life magic (healing and protective spells), Chaos magic (destructive spells), Death magic (weakness and undead summons), and so on.
I think it would be a good idea to add this feature in an expansion, especially if more spells are added to the game.