Would you prefer this version of Penance over the current one?
Every time you cast Renew, Power Word: Shield, Flash Heal, Greater Heal, Prayer of Mending and Prayer of healing, you get a Stack of Penance which lasts 15 seconds (stacks up to 10).
Penance - 20 seconds cooldown
Launches a volley of holy light at the target, causing damage to enemies or healing to allies instantly and every 1 second for X seconds, where X is the amount of Stacks of Penance you have.
So basicly what you would do with this is cast 10 flashes/renews/shields/PoMs and then fire off a Penance that lasts 10 seconds and hits/heals 11 times. I think it's an interesting mechanic that would allow you to keep the tank up during heavy damage fights, but you will also need to watch the raid's health while channeling as you might need to stop the cast and throw a quick shield to someone about to die.
Ofcourse the numbers can be tweaked to not make it too OP.