Hmm. Well, I have an idea, no idea how powerful.
"For every creature that dies, their attack is added to this shield's physical defense divided by 3 (maybe 4). Resets each turn"
Now, you get bonuses for killing creatures, but now mass killing reaps the most benefits.
Let's say there's 2 Black Dragons, and a Minor Vampire on the opponent's field, and the Black Dragons have 3 health each. You play RoF, killing all three. That's 22 attack, divided by 3, rounding down, is 7. This way, when the opponent attacks, they have to deal more than 7 damage to hurt you. It resets back to 0 when it gets to your turn. This way, it requires CC and preparation to make this shield extremely powerful in short bursts.
The reason I don't think it's OP of an idea, is because it takes a lot of preparation, and only helps in short, but powerful, bursts.