With the advent of the tenebrium skill, mages should have a Void spell equivalent, but that's a gripe for another day.
Now, I'd just settle for actually having tenebrium staves in the game...
But that's the thing, would a Tenebrium staff even be an upgrade for a Mage!?!
Depends on the stats, but mostly because:
a) Crackly evil staff of ominous purple doom
b) A certain quest in Luculla Forest
c) Crackly evil staff of ominous purple doom
Add in a black-dyed robe and hood, laugh.