There are three major disadvantages to staves.
The first is that they have less possible +skills than an Orb + Shield combo. However, staves can use runewords, which Orbs effectively cannot. Adding higher staff mods (like +5 Fireball) will actually overpower stave runewords. So really what is needed is for rare Staves to be able to spawn with +3 or +4 to Sorceress Skills.
The second is that they have significantly less faster cast than an Orb and Shield combo. I have a thread on this, but basically it should be pretty simple to add higher level Faster Cast mods to grant rare Staves up to 60% Faster Cast. I have that as one of the small changes that would help the game. Perhaps alternatively you could add Faster Cast to white Staves as that wouldn’t necessarily overpower the current runewords.
The third is that they have less defensive ability than a shield, both in less resistances, and in the lack of an ability to block. I suggested in my thread on Base Weapons that base staves should gain a “Parry” effect that would mimic the Dodge skill. However, that might be a bigger modification than you can implement?