In the Adventures in Middle Earth Player's Guide, there's a Hobbit Virtue called Small Folk (p. 109) that reads:
You have learnt how to gain an advantage in a fight from being smaller than most of your opponents. When you are being attacked in melee combat by a creature bigger than you (very often), you can take a bonus action to step into the same square as your opponent. You have advantage on your attack rolls against that opponent, until your opponent chooses to take a step back to attack you (which will provoke an opportunity attack).
How does this virtue work? Because "being attacked" means the opponent's turn but "bonus action" means my turn.
We think this bonus action could be used the next turn after you were attacked by an enemy.