Expanding on the suggestion I made above, what if you found some kind of item that had some effect, but could only work with skill X, Y or Z (semi-randomly generated). For example, uses your mana as a shield when you use Fire Strike or Magic Missiles. Or a spell that, if you use the cold imprison spell on your own minion (currently not possible but bear with me), will cause the minion to grow in strength proportionally to the time spent in the prison (an effect that wears off after a minute).
These items would make you *want* to mutate to another class because of the skill combos they'd provide.
Maybe this should be a kind of modifier on items. The important thing is that it's temporary -- at a certain point the combo item will be outclassed.