There are a lot of ways to make the spell more intresting, but it really depends on what they want to do with the different healers. IE, what does Holy lack, and what does it need?
They've said they want less fire-and-forget healing, so... Who knows.
What I think might be kind of interesting would be to have the lightwell function as a sort of toggled "turret"/pet. So, suppose they decide they want to make holy closer to holypal in tank healing. Let's imagine a fight like Vezax, so that there's "modes" of the boss. Lots of fights do this same thing, but that's a really straightforward one. Ignore the mana side.
Now, you could toggle the lightwell into "fill" mode while the boss is in its easy state. While in this mode, suppose all overheal is stored in the well, and all heal crits provide some stored amount whether or not they overheal.
Then, when the boss surges, the priest toggles the lightwell into "drain" mode, and then it does something to the priest's heals until it runs out. Perhaps the each heal you cast while in lightwell "drain" mode places a stacking hot on the tank for up to 10% of the heal value, and reduces the stored heals in the lightwell by the amount spent.
Then it becomes an intelligent ability, and you could use it in various ways (IE, pop well before the surge, to stack the hot sufficiently, or pop it during, if the surge is something that stacks a dot, like Toravon). Seems interesting to me.