I like the idea of empowering the Seal system again. Bringing back interactions between Seals and Judgment would be truly awesome. I'd rather see something like:
Passive Your Seals also cause your Judgment to empower you with an additional beneficial effect.
Judgment of Justice: Increases your movement speed by 10% for 20 sec.
Judgment of Insight: Heals a nearby friendly target for 3% of their maximum health every 2.00 sec for 20 sec. (smart heals the target of lowest health near to the target Judged)
Judgment of Righteousness: Up to 5 enemies around the target take an additional 10% Holy Damage from the Paladin's attacks for 20 sec.
Judgment of Truth: Increases your attack power and attack speed by 10% for 20 sec.
Because I think the Judgement/Seal interaction should feel more interactive for all three specs, but not be something too overly obvious to take.
I like the idea of introducing a new Seal, but Seal of Faith as is, is not the answer. There aren't really enough true dps abilities for Holy to make much use of this, and Ret would never shoot itself in the foot with a 30% reduction in damage for a smidgen more healing. And in truth, trying to balance the dps loss with big healing would just be flamed all day on the forums.
If you were going to make something relatively attractive to all specs, I'd say:
Seal of Faith
623 mana, Instant
Fills you with Holy Light, causing your Holy Power generators to grant Faith. Faith grants a bonus to the damage or healing of your next Holy Power consumer by 5% of the damage or healing done by the Holy Power generators.
Something where the bonus builds or stacks over time (like an Ignite)
I also like the idea of quasi-reflective damage in Divine Conviction. If they were going to go implement something like this, it'd probably be better as:
Struck by the Light (Holy) Passive When you heal a target, a nearby enemy will take damage equal to 20% of the healing done. (Higher health targets are priority)
Holy Shield (Protection) Passive Increases your block chance by 10%, and when you block, you deal 2000 Holy damage to your attacker.
Retribution Aura (Retribution) Instant. 3min Cooldown. 20% of the damage taken by the Paladin is reflected to the attacker as Holy Damage. Lasts 6 sec.
If the developers want something competitive with Atonement, the answer is probably the reverse-style Atonement above. Again, Holy doesn't have much in the way of dps abilities to make smite-healing practical, but it has alot of diverse healing abilities that could deal damage relative to their classically-burst oriented style.
I'd like to see a return for Retribution Aura. Just Devo Aura, it can come back as a short cool down.