We wanty! Imagine charging into battle with a Jackal shield. Sweet.
The shields should work like a dual-wielding weapon -- you should be able to hold it and fire a second weapon. By pulling the right trigger once, you would turn the shield on. Pull it again, and you would turn the shield off. You should be able to move the shield in the same way you would aim any weapon, so you could deflect incoming fire from different angles. Once the shield turns on, it would have a limited charge that would gradually deplete (taking hits would deplete it more quickly). You should also be able to give someone a charged melee attack by whomping them with the shield.
"Halo 3" needs to integrate more tricky explosives and traps. Remote and proximity detonated explosives, sentry guns and mines could add a whole new element to both the multiplayer and campaign modes. You could set mines by aiming at the desired spot and pulling the right trigger. The same system could be used for sentry guns. Remote detonators could be set the same way, but detonated with an additional pull of the trigger.
Sure, none of these things are groundbreaking new ideas for an FPS, but it seems like they are long overdue in the mother-of-all console shooters. A little minefield around your base would be nice in capture the flag. You could get really sneaky -- imagine watching enemies blown to bits when they try to board a vehicle you tricked out with a proximity mine. Surprise!