As it is right now the Inquisitor side of the Radaggan set doesn’t help much, and it might actually be detrimental compared to other class choices due to Possession being locked out in favor of Aura of Censure. The bonuses to AoC might have looked good a long time ago, but ever since we’ve had many buffs to all kinds of devotions and items (i.e. Yugol having target’s damage reduction %) and now it isn’t enticing enough to build around.
So my suggestion is to add Inquisitor Seal support to the set. Add some bonuses so we can reach 22/12 on it, perhaps some % damage absorption to account for not having Possession and - most importantly - make it green. I want to leave green circles all over the ground with my Deceiver.
The set could also benefit from some flat damage and casting/attack speed. My idea was to use Seal of Blight or Yugol’s Hunger, but the lack of flat damage and speed will probably prevent it from dealing good damage.
On a side note, some sets should have Inquisitor Seal conversions strictly for aesthetic/thematic reasons, like Voidsoul and Black Scorch Covenant. The graphics are already present with the Inquisitor Seal conduit affix.