I've been thinking about this a lot since starting priest, and part of why I've been going through making it in the way I have, a few levels at a time, is because I've not been sure how to do later levels until I get there.
I think a good capstone for D&D should zero in on the thematic and mechanical cores of the class, and improve them someway: Barbarian's Primal Champion is great at this for example. What makes a barbarian a barbarian? How strong and tough they are. How can we improve that thematically? Improve those stats.
Monk's Perfect Self on the other hand (correctly) identifies sorcery points as the core of the class, but doesn't quite hit the narrative for me. ki point recovery on inititive rest isn't a bad idea, but what does it actually tell you about the class, what's the narrative for recovering ki when you fight?
For Priest, there's one main point I wanted to zone in on with the class: The Customization. Priest has so many different options for how to build them, from Diety's Domain to Divine Calling to Method of Worship, I wanted level 20 of priest to relate to these somehow.
Initially I was looking at paladins, how they get their capstone feature from their subclass, in this case I was going to make Divine Callings give creatures that capstone, but that wasn't enough. Next I was going to make the capstone something all priests got that interacted with the different Divine Domains priests can connect to, but that also didn't quite hit the mark.
Eventually it struck me when discussing Mage Hand Press' Necromancer class earlier today. Necromancer's Capstone lets characters gain Lichdom from the core class, which grants a few benefits, but then the subclasses expand on this too. I could use this for Priest, and so came up with:
20th-level priest feature
As an action, you can become your deity or pantheon’s incarnation on the material plane for one minute. While you are an incarnation, you gain the following benefits as well as those dictated by your Divine Calling and Method of Worship:
At the start of each of your turns, you regain miracle points equal to half your spellcasting ability modifier.
At the start of each of your turns, you can change which domain you are channeling.
You gain immunity to necrotic and radiant damage.
You can use this feature once, and regain the ability to use it when you complete a long rest.
This feature already interacts with Divine Calling and Miracle points from the core class and grants two thematic immunities for a priest, but that's not all, as the feature mentions, Method of Worship and Divine Calling both also add features. Take Cultist for example:
20th-level Cultist feature
While you are an incarnation, the damage dice from your Vital Drain feature become d12s instead of d6s.
This feature interacts with the choices you've made for your character, and improves them in a way that's perfectly thematic. You're channeling the power of a god (or gods), of course it's only something you can do once a day, any more would be too much, but channeling like that is giving a power boost to you, a power boost that's dictated by the options you chose to get to this point!
