A tiny little mechanical hack I made to D&D was having players roll for Defense when monsters attack. I know it’s not much and it is not story related, but I like not having to roll dice in PbtA, so I imported that at least for the monsters attacks.
Mike Mearls had an Unearthed Arcana article were he puts this as optional rule, but the math was wrong. So the right formula is as follows: PCs Defense bonus is equal to their AC -10 (AC 16 = Def Bonus +6). And the DC to beat is the enemy attack bonus +12 (So if a bite attack bonus is +3 the DC to defend it will be 15, when rolling equal or higher you avoid/defend the attack).
I know it is not much, or story related as I said… but there you go.