I would go the other way and make plate armor very powerful that needs a separate strategy to take down. Knights were the tanks of the medieval ages and this could made an interesting challenge to win against one. Of course in exchange no regular soldier would have one, only companions, lords, named characters and the player could, which kind of makes sense. Regular people could not afford it in middle ages either.
I think the biggest problem in M&B single player right now is that while infantry playstyle is very fun on a mechanical level, it is borderline unplayable in single player due to two factors:
1.) enemy swarms you
2.) you will be shot down by archers/crossbowman. If you survive, you will be shot down again.
1.) can be fixed with minor AI tweaks: it should be possible to hit an ally in single player and the computer should avoid it by spacing out. Battles would be more like series of duels. This is as far as I know is also realistic, but either way that is not important - facing off multiple enemies that just gank you 10v1 (even if you have allies nearby, mind you) is kind of the opposite of fun, and otherwise game mechanics shine in duels.
2.) can be fixed by making plate armor invulnerable to arrows and bolts, except for headshots. Since only lords, companions and you can have one, this would not make archery pointless and you could still kill a lord by aiming for the head, but would make it possible to use a two handed weapon, since you had the option of not using a shield.
The problem is that using shield is not nearly as fun as blocking by weapon. The most fun method of playing should be both the hardest and most rewarding. Hbomberguy made a video about why dropping shield in Bloodborne improved the gameplay, and while this would be obviously a bad decision for M&B, they could still reward not using it. Of course, auto-block must be disabled. Not making it possible to disable it via menu - drop the option entirely. For those who worry about accessibility, there is still shield. Since plate armor would protect you from most of ranged damage, it would be kind of pointless if blocking was easy with weapons. The possibility to hit multiple enemies with a weapon is a step in the right direction, but it doesn't seem to be enough.
Plate armored targets must be either killed by specific weapons (like military pick or morning star), or must be knocked down. Knock down could work multiple ways, e.g. a hit while being off-balance could do it (like getting a hit after being kicked), hit to the legs maybe, or just being stuck by a huge hammer. This is definitely the hardest part to figure out, but I believe it is possible and would worth it.
Head would be still vulnerable, but I would add ducking. While it doesn't seem to be terribly realistic IMO, it is certainly within the boundary of physics and I don't think it would break suspension of disbelief. While ducking you could not block and getting a kick would knock you to the ground, making it a very risky move, but you could attack from any direction except from above, making it an effective way of landing hits in the middle of an enemy's attack, which would make it effectively impossible to block. Of course you could not aim upward (i.e. to the head) while ducking so damage would be somewhat limited, especially against other lords. This would make it a very high risk/high reward move. While you are on the ground you would get increased damage, making you vulnerable even in heavy armor. This whole part is needed because otherwise - at least in multiplayer - plate would be useless as opponents would simply aim for your head. Of course in multi I would only award it for kill streaks along with a two handed weapon. It would be such an epic reward and this way you could use two handed even in big battles where right now you just get shot immediately if you try.
While the exact mechanics should definitely be tweaked, this would make fighting lords and named characters feel really awesome. Right now when you slay them, you think "oh this was a lord, ok...". But if beating opponents with plate armor would need somewhat different tactics, it could be the highlight of a battle, an epic duel between the two leaders of the opposing armies. Maybe lords should also be able to give up if they are the only survivors or you would win even if he beats you - it doesn't really make sense to lose man/battle to a single enemy just because you have yet to master an aspect of the game.
