Homebrew retractable shield

Question

An idea that I had for a deployable shield as I didn't find anything satisfactory online.
It's simple, but I would like to know if it's balanced or if you see any weird interaction with it.

Retractable Shield (shield)

This (non-magical) shield appear as a complex metal armguard while unequipped and doesn't offer any benefits. At the start of your turn, you can choose to either don or doff this shield by interacting with it.

This could be a common magical item, a unique non-magical creation or a rare shield purchasable in some cities depending on the setting.

Answer 1

Homebrew that circumvents the limitations of your class features for free is unbalanced.

You provided some important details about your motivation for this design in some comments:

I wanted to play a barbarian/monk with the unarmed defence of the barbarian and the martial arts of the monk, that was what prompted me to search for this type of item.

barbarians can use a shield with their [unarmored defense], but monks can't use one with unarmored movement

The purpose of this item is simply to get around this limitation of the Monk’s unarmored movement:

Starting at 2nd level, your speed increases by 10 feet while you are not wearing armor or wielding a shield.

With a mundane shield, you must doff it with your action to “turn on” Unarmored Movement, essentially wasting your turn to do so. This retractable shield eliminates the cost of doing this by making it an object interaction instead of an action. This item was designed to circumvent a balancing limitation of one of your class features. That’s unbalanced.

However, “unbalanced” doesn’t mean the same thing as “wrong”. Discuss this with your DM and if you’re both okay with it, go have fun. The thing to consider regarding balance is that the limitations in class features are there for a reason, and getting around them for free can have unforeseen effects for game balance. But it might not. The only way to know for sure is to playtest it.

Answer 2

Existing items are always a good place to start when thinking about a homebrew item.

In this case, the DMG describes the Animated Shield as a Very Rare item that requires attunement and, as a bonus action, activates to "hover in your space to protect you as if you were wielding it, leaving your hands free" for a full minute. There are no usage restrictions, just the bonus action once per minute, and it doesn't even require proficiency.

It should be fine to keep those mechanics and then reskin the description of how it works to fit your desired "look", with DM approval. For example, we might describe this as not a flying shield, but as a metal gauntlet that can instantly deploy into a shield to block a blow and then retract immediately after. Or we could say it's an "Armor-caster" on your belt that fires out little flying armor plates to intercept attacks.

And if it's not quite what you want, making small alterations to an existing item is usually an easier "sell" to the DM than making up something wholly new. I'm not sure I'd remove the bonus action requirement, though, especially for a monk-barbarian, where your bonus actions are quite valuable every turn (as opposed to, say, a fighter or wizard, where most of the time they probably don't have much to do with their bonus action and waiving that requirement isn't particularly meaningful).