The problem with any sort of raid-wide Atonement spell is that even very short Atonements don't matter. You just Light's Wrath and bam, 2100% SP nuke and a 1050% SP heal to the entire Mythic raid. This means any spell like you guys are talking about has to have some very strong restrictions, whether it be in mana cost or in cd or in the form of some type of debuff on the priest or raid.