I did a bunch of research on icy-veins and carefully scrutinized and took notes on the talent trees, and came up with an idea for Archon that…well, I’m not sure how good a job I did:
https://www.deviantart.com/lookoutherecomestom/art/Ideal-Archon-Talents-Iteration-1-1027059034
They keystone talent provides an oscillating buff that first improves your Holy spells for a few seconds, then improves your Shadow spells for a few seconds. There are ways you can prolong the effect that buffs your main spec’s spells, and ways you can empower the other effect—in other words, a Holy priest’s Light Affinity will last longest if they cast some Shadow or Twilight spells during Void Affinity, but that Holy priest will be at their strongest briefly during Void Affinity. Contrariwise, a Shadow priest’s Void Affinity will last longest if they cast some Holy or Twilight spells during Light Affinity, but that Shadow priest will be at their strongest briefly during Light Affinity.
I won’t lie; that feels a little bit strange: creating a system that puts a priest at their strongest during a buff that’s the opposite of their spec’s flavor… On the plus side, though, this setup shows how The Light and The Void can play into each other, which sounds like the general kind of thing a Holy/Shadow tree ought to do.
There are also some Twilight spells that can benefit from and interact with this system both as a Holy spell and as a Shadow spell, including one Twilight spell that has no cooldown and can count as either a heal or a damage spell: Twilight Touch replaces Vampiric Touch for Shadow and Heal for Holy—a somewhat strange combination, but I think it can work. It’s worth adding that some of those Twilight spells are Twilight Word spells that can not only benefit from either Light Affinity or Void Affinity, but can even trigger your more powerful affinity the moment you need it.
Most of the new spells won’t contribute to spellbar bloat or rotation bloat, either, since they’re mostly replacements for your existing spells.
There’s one talent in the tree—an optional talent, mind you—that allows Shadow to heal or Holy to DPS if they want to. I try to add passives like those that allow for alternate playstyles. For that matter, there are a few places in my Archon tree where I try to give some Holy benefits to Shadow and Shadow benefits to Holy.
I like to think I did a decent job for my first try, but I won’t know for sure until I hear as much from priest players, Holy and Shadow.