I would personally really like to see taunt/interrupt spell, to help with the problems prot palas have with taunting, and a more reliable interrupt would be nice aswell, replacing the current HoJ one.
Since it causes damage it would be on GCD like earthshock and mocking blow. That coupled with a fairly long CD means it doesn't replace Righteous Defense as the main taunt. Again with the long CD and the fact that it will drain ret palies mana, means it's not an attack you will spam.
For holy it's nice with an extra damage spell in pvp.
My idea, stealing abit from the D2 spell:
Fist of Heavens wowtooltip
Fist of Heavens Rank 7
25 % of Base Mana 25 yd range
Instant 20 sec cooldown
Instantly blasts the target with heavenly
force, causing 1368 to 1392 Holy damage.
It also taunts the target to attack you and
interrupts spellcasting and prevents any
spell in that school from being cast for 2 sec.