Why does Goring Blade necessarily have to be in a combo? It could be on a 20/30 second cooldown (as opposed to 60s), which is something else you will have to pay attention to in your rotation.

What I wouldn't do is give it the ability to grant a damage buff which you can keep up indefinitely, just like Warrior or Dark Knight. We already have 2 tanks that fit that, so keep it on the side of Gunbreaker and original Paladin design.

If you want to give it more than just a hard hitting attack, you could attach a short, powerful DoT (15 seconds as an example), however, I probably wouldn't make it a DoT with a 100% uptime.

I would also want it to have some sort of AoE falloff damage, just so it can be used in trash packs. If you want to combine this with the DoT idea, it could be a more unique attack where the initial hit is the same for every enemy, however, the DoT is only applied to the initial target, that gives it an effective falloff damage that I believe would be unique across all jobs.

So, with everything I have said, how about this:

Goring Blade, 30 second cooldown. Deals 350 potency to target and all surrounding enemies. Additional effect: Damage over Time, Potency: 70; Duration: 15 Seconds. (Total 700 Potency on primary target, 350 on remaining, meaning it is effectively a 50% damage dropoff, the same as (almost) every other similar action for PLD).

This still gives you the really strong DoT application that the old Goring Blade used to have, whilst also keeping it inline with the newer PLD idea. It can also be used in AoE, so you don't have the weird awkwardness of a 'missing GCD' in the buff window, which comes with the effect that, your your 3 extra GCDs, that is your AoE combo (Total Eclipse > Prominence > Holy Circle) all fitting in nicely.