Blocking
Currently the only way to prevent incoming melee or range damage is via a shield and you wont be able to retaliate right away, but what if you were able to use other tools and or weapons to block with?

Imagine you're low on health and are being cornered by a vindicator. Your shield is broken due to taking the lethal blows by the axe of the vindicator. Right as he prepares to deal a finishing blow as a last resort you use your sword to block the incoming hit. Both you and the vindicator are stunned by the clash of your weapons, leaving you both wide open for an attack. Luckily your sword is swift and nimble allowing you to strike first taking out the vindicator so you may live to see another day.

Just like DPS (Damage Per Swing) there should be something for blocking like Block Stun (BS). This is how much time it takes (depending on the tool or weapon you use) to recover from a block back to neutral fighting position. To balance things even further there will also be a block cooldown which will take some time before you can use this ability again.
The tools I qualify for blocking are a pickaxe, shovel, axe and hoe. For weapons it will be a sword, spear, katana and greatsword.

Parry
A Parry is just like a block, but with a twist. If you manage to do a perfect block right at the moment it will give you a window of opportunity to do a counter attack dealing damage to the opponent in the process. The tools I qualify for parrying would also be a pickaxe, shovel, axe and hoe. For weapons it will be a sword, spear, katana and greatsword.

Shield Bash
To give the shield some extra use it can also be used as a weapon to charge right into an opponent if you decide to choose so.
To use this ability you need to sprint and use the left mouse button to initiate the attack. Something like this would be very useful to push away a group of zombies leaving them somewhat stunned in the progress. However you yourself are also wide open for an attack from behind.

Having some mechanics like this would make PVP & PVE in general a lot more dynamic and fun to experiment with.