Something we've done is to make the various wizard specialists actual specialists - that is, only a Diviner can cast Divination spells, only an Illusionist can cast Illusion spells, etc.
It keeps wizards form being a one-size-fits-all solution, since choosing a specialization actually specializes (i.e. limits) them.
It also means that the types of wizards we see is directly related to the type of campaign we are running; very few Diviners in a combat-heavy game, very few Evokers in a combat-light game, etc. There's been some grumbling, but most have enjoyed the variety and challenge.