Wrecktangle
Weapon (greatsword), uncommon

While wielding this greatsword, your AC increases by 2 and you are considered to be wearing a shield. You must be proficient with both greatswords and shields. https://t.co/yjgJhjhT3u

— James ☂️ Haeck (@jamesjhaeck) June 24, 2020

It may have an extra property, which increases its rarity.
▶️Steel (rare): You have a +1 bonus to your AC, and attack and damage rolls made with this magic weapon.
▶️Mithral (rare): You can also wield this weapon even if you aren’t proficient with greatswords or shields. ▶️Adamantine (rare): You have a +1 bonus to your AC, and attack and damage rolls made with this magic weapon. While wielding it, any critical hit against you becomes a normal hit. Additionally, whenever this adamantine weapon hits an object, the hit is a critical hit.

— James ☂️ Haeck (@jamesjhaeck) June 24, 2020

▶️Dragonbone (very rare): You have a +2 bonus to your AC, and attack and damage rolls made with this magic weapon. Additionally, when you take acid, cold, fire, lightning, or thunder damage, you can become immune to that damage type until the start of your next turn as a reaction

— James ☂️ Haeck (@jamesjhaeck) June 24, 2020

No. Unless your class or another element of your character says you're proficient with shields, you are not proficient with shields. i.e., the Fighter class's proficiencies: pic.twitter.com/tExTdTXfzB

— James ☂️ Haeck (@jamesjhaeck) June 25, 2020

What if the AC bonus was an on-hit reaction activation, providing +2 AC against the triggering hit. A permanent +2 AC almost feels a little OP. Or, you can forgo your Attack action to bring the Wrecktangle up as a shield, providing +2 AC until the start of your next turn. It's an uncommon magic item, not a normal weapon. Works totally fine.

— James ☂️ Haeck (@jamesjhaeck) June 24, 2020

Magical weapons such as this have been enchanted to counteract such awkwardness

— James ☂️ Haeck (@jamesjhaeck) June 25, 2020