To make the spec more appealing (than say, prot) as a DPS spec in arena, Retribution really needs some type of offensive utility.
The challenge in implementing this however, is to keep Retribution's unique feel by the introduction of new mechanics without making them too powerful.
Many Paladins suggest a form of mana spell cost increase as a unique idea to counter healers with the goal being more reason to bring Retribution into the arena.
Similarly I've come up with a spell that would function as a mana drain unique to Retribution.
Swift Retribution would be rolled into sanctified Retribution, and in it's place a 1-point PvP talent would be added.
Judgment of Zeal
Places a Judgment on the target causing all physical and bleed damage dealt to also drain mana = to
(damage dealt*(1/[MaxHP]))*2500. Drains caused by the Paladin will also restore 100% of the mana drained to the Paladin. Refer to individual seals for damage effects. Only one Judgment may be active per Paladin.
(The formula looks absurd with the "*2500", but what it is supposed to represent is % of max HP lost multiplied by 25, so it's really ((% of max HP lost as a decimal) *100), *25, but that's just redundant.)
The idea is to give Paladins a mana drain that is kept very weak in Raids and 1v1 encounters (still relevant to non-healers in 1v1 though!), but synergizes beautifully with physical damage dealing partners in arenas (classes that Paladins already mesh well with) to make for a threatening effect on healers that isn't yet another Interrupt or Mortal Strike effect.
Here is some hard numbers on the effect:
[On a target with 20000 HP]
[Paladin crits for 3000 with Divine Storm.]
(damage dealt*(1/[MaxHP]))*2500. --> (3000*(1/[20000]))*2500 --> (3000*(.00005))*2500 -->
(.15)*2500 --> 375
The number at the end of course doesn't seem very high at all, but when combined with a rogue partner or a Warrior partner, both dealing lots of physical attacks, it will most certainly add up.
As far as aesthetics go, it would probably be a Judgment with a purple icon.