Elemental Shield and Spirit Shield should become trainable spells.
Note: This argument mainly pertains to PVP.
In high-level pvp, Myth and Balance are both performing above most other schools currently. The goal of this change would be to deliver a minor nerf to Balance, as these shields wouldn't be exclusively Balance any more. And it would also be a nerf to Myth, as Spirit Shield is a soft-counter to Mystic Colossus, and Pierce.
There's the argument that this hurts school-identity, but in my opinion that argument is void. We can train Infection, we can train Tower Shield, we can train Satyr... I can go on.
Yes, I'm aware that TC variants of the card exist. You can't Aegis them, and you have to pack them in your side-deck. Both of these are reasonable disadvantages that could warrant wanting to carry the card in your main-deck.