Like the idea of death knight class and lich class, needs to be an optional path.

Think of this: the book of vile darkness clearly states that it could contain the way to turn the character into a lich or a death knight. Why not implement it with that in mind? It's a way to explore areas of play that are not normally used as options for players to play  as. If you guys have trouble finding how to work subclass options, use the lich options I have thought of for it: eldritch lich, demi lich, or lichen  lich. For the death knight: try getting  blizzard entertainment on board for using their "subclass" options: blood, unholy, and frost.