Well, it's a good deal more interesting than a simple global stat boost as I see it since it adds an element of randomness that a flat boost doesn't. It's in the middle ground between the worthless/boring talents and the useful/interesting ones.
I think it's important to keep the random element when tweaking the proc, but I'm not entirely sure that having it proc off of Medica would be all that useful. Recall that Sacred Soil is used *before* the AoE damage occurs and, on top of that, Succor can be used preemptively as well. WHM AoE healing are entirely different. Proccing off of a Medica cast only marginally less likely to end up with a wasted proc as it currently does, which is why I suggested attaching the proc to an ability you're liable to be using before a big AoE event, like Cure or Regen; even if you're liable to get some wasted procs, you're more likely to have a proc active at a time when it would be useful.How about say medica has a 25% chance of making your next cure III half price, similar to the scholars ability for a free succor. Since if you are placing sacred soils likely you want to prevent large AOE damage. Of course that might not always be useful either. You can have my suggested mechanic only have a 20-25 sec timer though when you generate a new stack it doesn't reset the timer.
A potential middle ground would be to have it proc off of Medica II, since it's an effective ability to cast in preparation for an AoE, thanks to the HoT component, much like Succor. I would probably have it proc off of the cast as opposed to targets getting healed so as to prevent requiring a ludicrously low proc chance to prevent a practical proc chance being too high (even a 5% chance per target healed by the initial component on 8 targets would amount to a 33% chance to get the proc).
Reducing the damage just makes it less likely that you'll get the heal off without the debuff that provides the healing. Even if you can use it more often, you're reducing the "proc" chance. If you want to have the heal be purposeful, it needs to be predictable (which it isn't now because you have to be crazy lucky to get it at *just* the right time), which is why I say that the debuff needs to be present on the ability by default. Reducing the CD is just trying to turn it into more of a DPS tool, as opposed to a more reliable self heal, which is what I'm inferring your intent to be.You can reduce the potency on mercy stroke for CC if it's that worrying about the damage, keep it 40 secs. (40 sec CD, 100 potency/200 potency for war) Most of that was put it to make mercy stroke more interesting as a CC as well as improving it for healing. Honestly the main use I would've thought the healing portion of the ability is during add phases to save the healer some mana. So if it put a debuff on the enemy at least then it could be used to actually gain the health back, instead of chancing against the dps.