Such an open ended question. Tempting. But it depends on how radical you want to be.
Truth is, there is a lot that can be improved. And a lot that works well as is. Issues with the class that players seem to have a problem with are Inquisition, AoE damage, ramp up and the feel of certain moves such as HoW.
My own thoughts are simple:
Inq needs to be removed or redesigned. It is intended to add needed complexity, but in truth, I don't feel it succeeds. Personally, I'd remove and work the
complexity into the HP system which should be expanded upon. I just have the feeling Blizzard got bored wih reworking the class and left it alone this XPac. No Inq also helps with ramp up.
SoR should be removed or redesigned. I don't think the AoE Seal mechanic works well. The damage component, such as it is, should be added into the various AoE moves to make them feel meatier.
HW (old style - I do like the new 'melee' version Prot has tho), Exorcism and Consecration should be baseline. They add flavour, a provide scope to boost AoE damage and vary the rotation. I'd consider making DS and TV baseline and MELEE abilities (not spells affected by silence) but while I feel such as sytem woudl give the class a very decent baseline toolkit, it removes much of the variance between specs.
The first talent tier should be reworked. PoJ should be baseline, but with an active component - when active, increases speed by X% for Ys. Passive PoJ aspect inactive for Zs afterward and sprint has a X min CD. Give the class some baseline mobility.
DP should be baseline. Give it an ICD or something, but the talent really helps fill out the rotation. A filler move of some sort may still eb helpful to keep the rotation flowing.
I still miss Auras.Don't know why.