thats an interesting thought, a new mechanic could be rather good, but rather than a copied mana bar determined by healing someone how about making radiant users gain a "sanctity" count?
each enemy slain or ally healed by radiant damage slowly adds to the count(+0.1 count?), this count does two things,
1: the count directly bonuses all radiant effects (healing or damage) by like 0.1 per count? just because a full +1 would be a tad OP likely, but it needs testing, that or it could be a 1% bonus sort of thing (but that is even weaker and would not help early game healers at all really)
2: the radiant user shines light in a radius based on the count (probably 0.1 again), anyone in the light gains HP once a second equal to the count - how many blocks away from the radiant user they are, a similar thing happens as damage to enemies in the light but at X 0.1 the healing power of the light
this count goes down by 1 every like 10 seconds in which it was not further boosted
it also goes down equal to the damage taken when a radiant user is struck, however when this happens anyone in their aura of light gets healed for the damage taken by the radiant user up to the amount the count holds
the radiant user does not passively heal themselves, however they get 0.1 X the healing they do to allies within their light and X 0.1 the damage dealt by the light
this would also help balance the weakness the class currently has
they can still start out way way below par, but it would slowly build as they "cleanse and sanctify" the world, thus letting them become quite decent at the cost of likely getting wiped out early
just a thought, but it seems that this would work pretty decently
