Here's a simple idea - hitting Block at the instant an attack is about to connect grants some sort of bonus. This could take the form of an automatic, unblockable riposte (perhaps at significantly less damage than a regular attack, to stop players abusing the system), breaking an enemy's combo (I'm thinking specifically of Alpha Wolver's multi-bite attack, but there are other examples), an increased shield-bash knockback for the attacker, or perhaps simply negating the damage done to the player's shield, allowing skilled players to avoid (or at least delay) shield breaks.

Enemies very obviously telegraph their attacks, but different monsters have vastly different attack speeds and animations - consider how easy it is to predict a Wolver strike after the flash, compared to a Griever - so this tactic wouldn't be applicable 100% of the time, nor do I think it should be. It's also obviously not without its dangers - if you mistime your block, you take full damage from an attack you might've avoided by turtling. As I say, it's a small touch, but could inject a significant level of dynamism into combat without drastically altering its existing flavour.