Iron Heroes and 4e seemed to have a feature where the magic user could cast an attack spell repeatedly (no limits) in much the same way a fighter could keep swinging his sword, or an archer could keep shooting arrows.
The gist was, a mage could cast magic missle 1-3 times per day for 2-5 damge, then be screwed. Whereas everyone else could keep doing things in their domain of cool.
To improve this, I'd consider a rule where a magic user can choose a spell of "rapid" casting time, that they can cast without limit (1 per round, we're talking no daily limit). If it wasn't too unbalanced, I'd consider letting the magic user choose 1 spell per caster level (thus at 3rd level, have 3 unlimited spells).
By "rapid" I mean spells with instant casting times (within the round's action). This should lock it down to useful spells, hopefully not too abusive.
Math would need to be done, to test how comparable the best spells being cast every round would be to a fighter of the same level..etc.
However, something in the vein of more spell casting of certain types would help wizards, especially at low level.