Personally I think the idea of creating [Fortified Mana Shield] for Mage class is cheap. It's just the same Mana shield just a little bit stronger.
I'd like to suggest 3 new "Mana Shields"
Frost Globe
Electromagnetic Field
Flame Shield
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Frost Globe
Blocks damage with 50% of max MP (Like Mana Shield)
Attacking monsters ASPD and MSPD is reduced by %
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Electromagnetic Field
Blocks damage with 50% of max MP (Like Mana Shield)
Stuns attacking monsters with % success and deflects 50% of dmg back to the attacker (Like Knight's shield)
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Flame Shield
Blocks damage with 50% of max MP (Like Mana Shield)
Damages attacking monster by # and adds burning effect which deals # damage every second
And for Mana Mastery/Mana skills tree I'd suggest keep Fortified Mana shield, but with higher amount of blocked DMG.