Has the same durability as a normal shield and works basically the same. It could look like a very small disc made of compressed iron that "opens" when you use the shield button, making a holographic shield around it. This could make for better visibility, as it is very small it could occupy very little of your screen until you block with it.

Armor upgrade

Increases shield durability.

Item life upgrade

The shield can auto-repair with an air cost.

Volume upgrade

The shield has more air capacity.

Security upgrade

With a command hotkey, the shield could project a wall of holographic blocks that protects the player. It lasts until the player presses the hotkey again and costs a lot of air while it's up. Entities (including players, projectiles and items) cannot pass through the holographic blocks and neither can liquids, however they can be broken very easily. The size and cost of the wall increases with the amount of upgrades.

Dispenser upgrade

When the shield blocks damage, it can release a vortex ring, similar to the one produced by the vortex cannon. The power and cost of the burst increases with the amount of upgrades.

Inventory upgrade

The shield uses some air to deploy a holographic version of a block, which can be interacted with just like the regular version of the block. The block can be very easily broken, and (if it wouldn't be very hard to program) mobs and players can pass through it. When the command hotkey is pressed, a GUI opens, and the player can select the block they want to deploy. With only one upgrade, there are only two options, but with more upgrades more options can be unlocked.

• 1 upgrade: The player can deploy a holographic crafting table or a holographic barrier block. The barrier block cannot be passed through by mobs or players, and has the same properties as a block from a barrier created by the dispenser upgrade.

• 2 upgrades: The player also has the option to deploy a holographic furnace and a holographic ladder, which can be climbed.

• 3 upgrades: The player also has the option to deploy a holographic chest (with a lower inventory capacity), fluid tank (with a lower fluid capacity) and anvil (doesn't fall or damage mobs). If the holo-chest is destroyed, its contents are dropped on the ground. If the holo-tank is destroyed, its contents are destroyed.

• 4 upgrades: The player also has the option to deploy a holographic ender chest that can access the player's ender inventory. This has a higher air cost than other options.

I'm not opposed to the concept of a Pneumatic-powered shield, but some of the ideas (basically all the holographic stuff) feel quite out of place for a shield.