I have to say. It seems pretty odd to reduce the "what does a fantasy priest in a mythical setting successfully proselytizing yield (?)" question down to coinage (and away from the thematic questions of related to providence)?
How about first answering the below questions:
* “What does increasing followers do for my faith when it comes to both setting and situation?”
* “What does that setting/situation change do for/against specific characters? (PCs and important Cohorts)?”
* “Who does it moralize and who does it offend?”
* “Does it embolden the spread of further proselytization like social wildfire?”
* “Does this invite a collision of belief systems or even holy war?”
Then ask the question:
How can some/all of this be systemitized so that it has mechanical heft that intersects with the priest's subsequent decision-points as they pertain to the faith?”
One answer can be found in Blades in the Dark Cult games which engage with all this stuff the following way:
* You name your deity and give it two thematic tags which manifest in play when you deal with supernatural complications related to your deity's manifestation such as
Alluring or Cruel.
* You gain a group of Adept Cohorts (ritualists, people that can attune to the supernatural, et al) that increase as the faith grows (your Tier increases) and your terrible God/dess becomes ever closer to manifesting fully.
* You choose a favored Operation type between
Acquisition, Augury, Consecration, Sacrifice. These have mechanical benefit as well as thematic direction and feedback loops for play.
* Thematic xp triggers are carrots for play that both (a) complicate the cultists lives and (b) reward them for engaging with this. Example:
When you play a Cult, you earn xp when you advance the agenda of your deity or embody its precepts in action. Instead of hunting grounds, you have sacred sites that you use for your operations.
* The cultist PCs gain mundane upgrades throughout play like a
Ritual Sanctum in their Lair which serves as an arcane workshop for occult practices and rituals or
Ordained which gives them an extra Trauma box because their minds have been opened and conditioned to the great and terrible things beyond the mortal veil.
* The Cult itself (the faith here) is its own character which is built out with all manner of thematic abilities that augment the Cult's pursuit of spreading the faith and bringing their Forgotten God/dess back to Duskvol (or at least making it manifest to terrible ends, profoundly perturbing the status quo in the interim). Such as:
Conviction
Each PC gains an additional vice: Worship. When you indulge this vice and bring a pleasing sacrifice, you don’t overindulge if you clear excess stress. In addition, your deity will assist any one action roll you make—from now until you indulge this vice again.
What sort of sacrifice does your deity find pleasing?
* The Cult's Claim Map gives them thematic, mechanically potent and linked goals to pursue as below:
* The setting itself is deeply bent against your faith spreading and your God/dess manifesting as (a) there is a state religion (The Church of Ecstasy) dedicated to the Emperor (therefor what you're doing is heresy), (b) they are allied with powerful figures (the City Council, the Leviathan Hunters, the Spirit Wardens) but they also have enemies that you can ally with, and (c) there are other competing cults of demon and deity alike that you will compete with for spread of your faith and for ascendency. The Faction game will find you ticking +1s or -1s with these groups regularly so that you'll make allegiances of temporary convenience and enemies to go to war with and put down. You'll be dealing with multiple Faction Clocks for the machinations of all of these parties during Downtime and for the waves you make/damage you do to the setting. These Clocks will tick each play loop with each having entangled consequences for your Cult, your deity, your membership, bystanders, and the brick/mortar/filth (actual and moral) of the place you inhabit.
Dogs in the Vineyard handles this another way. Torchbearer another. Dungeon World another. Stonetop yet another (like Dungeon World but with a discrete playbook for your Steading and related moves just like your Cult playbook in Blades).
So 5e may want to ask and answer at least some of these kinds of thematically and logistically appropriate questions, systemitize them with conjoined mechanical and thematic heft (so decision-points are all fed by this stuff, intersect this stuff, and churn out this stuff) for a game interested in engaging with that sort of material in a meaty way.
I would think for 5e, something like the Blades route or Stonetop route with the Faith being an actual PC sheet and giving each member of the group some kind of gestalt abilities, attendant goals w/ thematically and mechanically robust rewards, and then attendant responsibilities and fallout related to it attaining those goals and spreading your faith (in a world that inevitably has supernatural competitors for that exact same space). And supernatural and logistical consequences should be always lurking, transparent (table-facing), and potent so the players know what is at stake as the push at the world and the world pushes back.