That's my bad - I understand the options better. If it's simply a passive that comes with the trinket, that's great. I don't like the idea of having a passive added to the trees that you have to spend a point on.
I'm with you on the power aspect, and I'm on board for one trinket that really juices up radiance or Interdiction. I think option D is pretty good and I wouldn't be upset if that's how we do it. Needing to use a spell cast from another spell level feels a little off to me, though I'm with you in it being a simple implementation. I understand the experience and power level of getting all the upgrades in 1 cast at higher levels, but if it uses a spell cast, we then have to design these to compete with other spells at that level. If it isn't compelling, then it won't be used, if it's too compelling, then it causes a similar problem as the current "too good" priest spells. Designing "new" abilities feels out of scope for this mod vs. updating existing ones.
Something I like about Radiance is that it's per encounter and can be used every encounter without costing the use of something else. It's balanced around that, but with some investment in talents in PoE1, it could become really strong. In this case, with some investment in a trinket, it can become stronger, but doesn't cause the extra problems of competing with other casts. If we can turn interdiction into a similar ability, I think that's worth it's own trinket.
What about something more like the Undying Burden, in terms of granting another cast of the same per encounter, but also juicing up all casts? Would that be too powerful? Choose two of these to keep?
Glowing Sermon Page (St. Rumbalt flavor text)
Brilliant Radiance: your Holy Radiance now deals a small amount of fire DMG to all enemies (Vessels hit are knocked down)
Gain ability: Aggrandizing Radiance - once per encounter, duplicates almost all the effects of Holy Radiance (including burn DMG, disposition check, etc.), but grants +4 to each attribute to the priest for 10 seconds and does not heal allies.
Gain ability: Inspiring Radiance - once per encounter, duplicates almost all the effects of Holy Radiance (including burn DMG, disposition check, etc.), but gives allies +10 Acc for 8 seconds
For Interdiction, I think a separate trinket is better. It's always been a talent choice, and it fits with a different style of priest than Holy Radiance. We can combine the PoE1 traits along with some "style" boosts to similar abilities via PL buff.
Singed Scrap of Cloth (flavor text around Durance)
Searing Words: +5 PL with Condemnation spells, your critical hits with Condemnation spells lower target Burn AR by 4.
Gain ability: Painful Interdiction - once per encounter, as interdiction, but also Weakens targets, +5 Accuracy (10 total with PL boost)
Rough around the edges: -5 PL with Inspiration and Restoration spells and abilities