This has already been asked for and they said they don't want to do it. Instead they said they'd consider adding smaller shields.

That said I'm really on the fence about invisible shields. If you're one of those people that goes shirtless or whatever then it would look weird blocking a sword with your bare arm, but if you're in armour it's entirely possible to block a hit with say, a gauntlet. And shield bash and shield swipe still make technical sense, you're just smashing them in the face with your arm or pushing their weapon out of the way.

It can also be argued that the shield is still there, it's just invisible which would actually give it tactical advantages as the enemy would be caught off guard by thinking they're going to hit you and instead hitting an invisible wall.

I guess I'm personally for it. The main reason, and I'm sure most people's main reason, Lightning glamour looks weird with a shield since she didn't use one.