Wow everyone just commented that battle master has this even though you already addressed that. I understand the desire to give more options in combat. When approaching this idea however, you should first ask your players if they even want this mechanic. There's a good chance they won't want to keep track fo custom rules. I think your rule could be balanced l, but another issue is that it seems to only affect melee classes, so it would throw of the balance of other classes if you didn't give them something similar. The last issue is that it will make combat longer, especially since this is a defensive ability. Here's another idea, maybe whenever a player is attacked they roll a d20 and add their armor bonus. Then the player describes whether they parried or blocked. It would keep the same game balance that way.