The only thing that should potentially restrict spellcasting is a Shield because it's cumbersome and slow to unequip. But even with a Shield I think you can put away your weapon as a Free Action to cast a spell, and then redraw your weapon as a Free Action on your next turn, only losing a potential Attack of Opportunity. Making that automatic in a video game makes sense and that potential AoO is not a game breaking exploit.
That said, I think it would be cool to give spellcasters some mechanical incentive to use two handed weapons instead of a Shield, because it just makes sense if you need free hands frequently. Shields are so powerful they're a no brainer for Clerics. There is no "spellcaster Cleric" with a staff, they're all much better off with that +2 AC from shields.
A Shield should also make archers slower to switch between melee and ranged. Or conversely, using a two handed sword with a Bow should allow for faster switching. Instant weapon switching is really useful in BG3 where battlefields are always wide open spaces. 5e mechanically already has a restriction for Shields. Equipping a Shield takes an action. I think that should be enforced in BG3. The previous BG games didn't let you equip bows in quickslots if you had a Shield equipped and while being a crude hard restriction, I thought it made sense and diversified character builds.
Shields are very powerful and should have some tradeoffs at least.
+1
Indeed it is, and if a rogue in a party I'm running tries to backstab a character with a tower shield on their back, they're going to fail. The simple reason being that the shield isn't some nebulous image, it's a solid object, that is there. If their weapon wouldn't cut through it like butter from the front, it's not going to cut through it from behind either. That's a lot simpler than trying to explain, as I initially asked when this came up, where did that shield go?
Rogue backstabbing someone doesn't mean they literally stab the person in the back. It means they sneak up from behind and get a stealth attack. They catch their target off guard. It can mean jumping up on their back and slitting their throat. It can mean reaching around and knifing them in the side or in the chest. It can mean stabbing them in the back of the neck. It can mean bashing them in the back of the head.
Rogues don't backstab in 5e; they perform "Sneak Attacks." Rogues in 5e "know how to strike subtly and exploit a foe's distraction," which happens if they have Advantage on the attack OR an ally is adjacent to the enemy. Nothing in the rules requires them to be behind their enemy or target their back, or even references the enemy's back.
Additionally, facing isn't a default rule in 5e. An enemy's pawn that is facing you is treated exactly the same as them facing away from you.
All put together: shields on backs don't provide a benefit to AC or against sneak attacks in 5e. You can rule that as homebrew, sure, but WotC specifically made decisions when designing 5e. And such a homebrew should come with a penalty; otherwise having a shield on your back is strictly better than wielding it and why wouldn't everyone do that for the free +2 AC??
