My problem with your changes is that they'd just be a massive powerup for mages. They'd have A/B/S just like they do now, except they *ALSO* have a built-in "barrier-like" spell for those times when barrier isn't available. That's a little too much, IMO.

Under the system I proposed, the point of aura and shield is to account for those times when you're stuck in the "cooldown" timer from the barrier-like spell.

Aura + shield approximately equals barrier.

So, the days of a mage having full a/s/b would be gone. However, a mage would be guaranteed to at least have the equivalent of barrier on tap at all times (subject to the cooldown timer). If he also used aura and shield wands, then he could have the equivalent of barrier (i.e. either the built-in spell or aura+shield) up for a very long time, such as during a raid situation.