Hi there!
Season of Discovery is great and I like that it brought attention to some of the poorly balanced specialisations and even gave us new ones 
Still, there are some niches that haven’t been touched and one of them is smite priest, which I personally like and played it in late Classic and in TBC against all odds. But with the opportunity of SoD or whatever the future brings, I think it could get a little attention and some polishing. Here’s my idea.
The general role of smite priest would be a holy based damage dealer, who can also provide a bit of offhealing and also work as an emergency healer if the sitatuation is dire. My idea of making it work is based mainly on adding a bit extra stuff to already existing runes, but also creating 3 new ones and making couple baseline changes. Let’s start with that:
Baseline:
- Make Holy Fire periodic damage effect able to crit, without a need of any rune. It wouldn’t affect shadow priest as they don’t use that spell.
- Buff the talent Searing Light - it is in the middle of holy talent tree and with 2/2 points it increases damage done with Smite and Holy Fire by 10%. The spec suffers from dealing rather low damage, so doubling that talent buff would help with that, so make it 20%.
- Inner Fire could give spellpower buff, like in WotLK. To not make it a boring, flat amount, make it scale with other spellpower priest would have and link to the number of stacks. Have it give 5 spellpower + 1% of your current spellpower buff per stack. That would mean, with full 20 stacks and 500 bonus spellpower on priest, Inner Fire would provide extra 200 spellpower. With 1000 bonus spellpower, it would be 300. If those nubmers are too high, it can be easily adjuest by lowering the numbers in formula.
Now, the runes:
- Wrist: Surge of Light. Only rune that already exists that has something to do with smite, makes it instant cast after a crit with smite of flash heal. Make it give instant smite after any critical non-periodic spell cast.
- Chest: Serendipity. Add an extra effect: casting smite, flash heal or binding heal decreases cast time of next holy fire by 0,25 seconds, stacking up to 2 times; casting lesser heal, heal or greater heal decreases cast time of next holy fire by 0,5 seconds. It would provide nice synergy between two of the main spells in the smite priest kit and also make it non-punishing if you would need to cast a heal.
- Hands: Penance. Add an extra effect: casting penance for full duration increases your holy damage dealt by 10% for 15 seconds. Penance is used mainly for healing, but it can also be used as a damage spell. This would give it a spot in the rotation. Also, the healing usage of penance would also provide the buff.
- Waist: Empowered Renew. Add an extra effect: whenever you heal with renew, it increases your crit chance with holy spells by 5% for 10 seconds, stacking up to 4 times [doesn’t count when it’s overhealing]. This I think would be interesting, to include a healing spell in your damage rotation. Keep it rolling on the tank, or maybe life tapping warlock to keep the buff?
- Head: Eye of the Void. Add new ability for the summoned eye: Curse of Vampirism: reduces holy resistance by 75 and increases holy damage taken by 10%, also sometimes heal the target of your attacks by 2% of their maximum health. This is controversial, using a shadow oriented ability, but I think adding a new type of debuff that would buff smite priest (and also ret paladins) would be cool, with the unique effect of making the boss sometimes heal the tank a little.
- [new rune] Feet: Improved Holy Fire. Perodic damage crits from holy fire cause an effect of holy nova on the target (internal cooldown of 1 second shared among all holy fire dots active at the same time). Add some extra flavour to holy fire spell, giving you a tiny extra aoe and offhealing.
- [new rune] Back: Spiritual Infusion. Increases your target’s (can be self cast) spell damage by 10% for 15 seconds, also passively increases your spellpower by 15% of your spirit (shares cooldown with power infusion, stacks with spiritual guidance). Smite priests as they exist now can be build in two ways, either 31/20/0 or 26/25/0. First build gives Power Infusion and Force of Will (5% extra damage and crit chance), second trades that off for 25% of spirit becoming bonus spellpower from Spiritual Guidance. That’s a bit rough trade off, so this rune would mitigate it and give you a budget Spiritual Guidance if you go for Power Infusion, or budget PI if you go for Spiritual Guidance (and even make it stronger in that case, as I think it’s the less appealing choice right now).
- [new rune] Legs: Holy hammer. You summon a holy hammer on targeted area, dealing moderate aoe damage upon summoning; the hammer remains in that area for 20 seconds and attacks with basic attacks, each attack causes a holy nova explosion, also casts smite every 5 seconds; hammer prioritizes target you last casted smite on within his area of operation. New ability with maybe 45 seconds cooldown, working as a guardian (so not controllable like a pet), providing you with a bit of aoe and extra damage as long as your target remains in the area. The size of the area could be like Rain of Fire. Hammer would be untargetable, working kind of like Dancing Rune Weapon of Blood DKs.
So here’s my idea, I know it’s purely academical, but I had it and wanted to share Thanks for reading!