I think a nice way to keep Divine Storm a signature ability - one that doesn't get thrown into the backpack for AoE fight use only - would be to make it do different things in different scenarios.
Boss Cleave Style:
Divine Storm - Requires Melee; Instant Cast; 10 sec cd. Instantly strike up to 4 enemies, causing weapon damage evenly spread, each additional target increasing total damage by 40%. The Divine Storm heals up to 3 party or raid members totaling 30% of the damage caused.
Dual Style:
Divine Storm - Requires Melee; Instant Cast; 10 sec cd. Instantly strike all enemies within 8 yds, causing 100% weapon damage against a single target or 50% against multiple targets. The Divine Storm heals up to 3 party or raid members totaling 30% of the damage caused.