Rebalancing redux so that you get more of it for having only 4 shields would be nice, but here's another idea for pushing up their power. Each needs affinity 9 or higher:

Enhanced Shield of Flames - Searing heat occasionally scars the one fighting you.

Enhanced Shield of Waves - Much longer lasting (for water breathing). Gives extra redux resistance to all elemental damage.

Enhanced Shield of Earth - Gives resistance to piercing attacks. (already has a nice redux bonus, doesn't need much more)

Enhanced Shield of Slime - Hard strikes produce gouts of grease onto foe. Gives total immunity to bashing/lashing/whatever.

Enhanced Shield of Wind - Even more immunity to kicking, grants flight.

Enhanced Shield of Ice - Gives resistance to slicing attacks and occasionally deflects blows with it's hardness.

Enhanced Shield of Lightning - Parries metal weapons occasionally.

My other idea was spells that are only available to people with a high affinity in an elemental. Each spell would attempt to make up for a weakness on element has - such as affinity 9 in ooze would give you an acid storm spell - like pillar of lightning, unshielded people are whacked. Though it wouldn't be nova/pillar, it'd be nasty.