Well if it HAS been done a thousand times before, and never managed its way in the game, that's probably because Zanzarino doesn't like the idea... anyway here's a list of shields and the effects I would have them have.
Dissipation Shield: 0/N
Dissipation Shield's max hp are equal to twice the amount of entropy quanta you have.
Bone Wall:
Bone Wall transforms in an equal number of Skeletons when animated.
Gravity Shield: 0/10
Gravity Shield cannot be harmed by creatures with more than 5 hp.
Titanium Shield: 0/40
Thorn Carapace: 0/10
Creatures that deal damage to Thorn Carapace have a 75% chance of being poisoned.
Fire Shield: 7/1
Ice Shield: 0/10
: freeze
Reflective Shield: 0/10
Immaterial.
Hope: 0/N
Hope's hp are equal to four times the number of light emitting creatures you have in play. Immaterial.
Fog Shield: 0/10
Fog Shield has a 40% chance of being missed.
Wings: 0/10
Wings cannot be harmed unless by an airborne creature or by a ranged weapon.
Procrastination: 0/10
Creatures that damage Procrastination are delayed by 1 turn.
Dusk Mantle: 0/10
Dusk Mantle has a 50% chance of being missed.
Dimensional Shield: 0/N
Dimensional Shield's hp are equal to the number of turns it will last. Dimensional Shield is immune to damage.
This is just a first idea, and will be subject to modifications. Probably in the shields' hp at least.