I think I’d prefer to see the mastery change completely. An idea would be to have a smart heal component splitting additional healing to nearby wounded targets on direct heals.
E.g. 8%-25% of your direct heals splash to up to 3 targets within 6 yards of their target, dividing the bonus healing by the most wounded.
So let’s say you have a tank and two melee DPS, the melee are at 90% and after the heal lands on the tank, she will be at 75%. The lost health percentage total is 45%. Your mastery is up to a 12% splash, so the mastery procs to heal the tank for a bonus ~6.7% (25% health missing / 45% target pool missing health percentage), and the two melee for ~2.7% (10% health missing / 45% target pool missing health percentage).
Since shaman healing can already be strong AoE, if there’s fear about the splash being too enabling, the division and language can be changed to something like:
… dividing the bonus healing to the primary target first and then by the most wounded.
In this same situation, the bonus healing for 12% mastery bonus would be a total of 9.3% bonus healing to the tank (6% + 6% * 25% missing health percentage / 45% total pool missing health percentage) and a smaller 1.3% splash to the nearby melee damage targets.
The effect would prioritize the 3 most wounded, so if the tank were at 100% after the procing heal lands, the first 90% melee would be the “primary target”, obtaining 9% of the splash healing (6% + 6% * 10% missing hp / 20% total pool) and the second 90% melee target would get the remaining 3% splash.
Trying to hard limit to “other” targets would not be desired! If the tank was isolated or no other nearby targets were injured, she would get the full bonus of the mastery (if zero, 100% primary). Otherwise gaining mastery in isolated healing scenarios goes wasted.
The coolest thing with this design would be casting chain heal and seeing a bunch of big and little heal procs flying out, sometimes healing the same targets with the splash.