My first rules foray into 5e; I'm going to be running a campaign at the library in March, and wanted to dip my toe in. Since Pantheist Clerics are a thing I've been writing about for 20 years or so, this seemed a good place to start, and aligned with a conversation folks were having on the Playground.
Some priests do not choose a particular deity to worship, and instead embrace the full pantheon of their people. Unlike more specialized priests, these clerics can perform ceremonies for the entire pantheon; they may pray to Kelemvor at a funeral, invoke Lathander upon a birth, bless soldiers in the name of Tempus, and open a harvest festival in the name of Chauntea (or Waukeen, or Llirra, or all three).
Being a Pantheist cleric can be a difficult balancing act, especially for an adventurer. Depending upon their alignment and personality, a pantheist cleric will undoubtedly have certain deities whose philosophies they favor, and others whom they are less sympathetic to. However, pantheist clerics serve an important role in the worship of many deities; while it may be somewhat illegal to worship deities such as Mask, Chemosh, or Iuz, a pantheist cleric can invoke them as needed, often serving as a link between worshipers and their deity in out of the way places, or serving an isolated community's varied religious needs.
Pantheist Doman Spells
1 Bless, Sanctuary
3 Calm Emotion, Gentle Repose
5 Dispel Magic, Tongues
7 Divination, Freedom of Movement
9 Commune, Hallow
At 1st level, you become proficient in Religion, and your proficiency bonus is doubled when using this skill.
Channel Divinity: The Will of the Gods
Starting at 2nd level, you may use your Channel Divinity to inquire of the Gods about a course of action. This is equivalent to the spell Augury, with its attendant difficulties for overuse.
Channel Divinity: Pantheist
Starting at 6th level, the Pantheist can draw upon their relationships with the gods in a more intimate way. They may spend a use of Channel Divinity to consider use Domain spells of any other Domain available to their pantheon as Domain Spells for 10 minutes. The cleric must still expend spell slots for these spells as normal, but they do not need to have them prepared.
Familiar with the blessings of many gods, Pantheist clerics receive a number of additional cleric cantrips equal to their Wisdom modifier at 8th level.
Friend of the Gods
Beginning at 17th level, Pantheist clerics are able to request the intervention of many different gods. They may successfully request divine intervention a number of times equal to their Wisdom modifier in any given seven day period, with the restriction that they must request the intervention of a different deity each time. If, for example, a cleric of the Faerunian pantheon with a 20 Wisdom successfully requests intervention from Mask, they may successfully request divine intervention only 4 more times before 7 days are out, and then, not from Mask. On the 8th day, they would regain 1 use of Divine Intervention (the one they had previously used for Mask), and could invoke Mask's aid again, if they wished.
Spells: I think the selection I made is fairly strong, partially because some really useful but situational spells are in there... Bless, Sanctuary, Dispel Magic, Tongues, Freedom of Movement, and Hallow, especially. But, it is not QUITE as useful as some domains, because all are Cleric spells, so they're just slightly more cleric-y clerics, rather than having any not-quite-cleric skills. Another part of the idea (and why Cure Wounds was not included) is because I didn't want to step on anyone's specific niches in spells... Sure, they share Bless with Life Clerics, but they're not healing-focused. They share Dispel Magic with Trickery, and Freedom of Movement with War, but they don't step on any given core domain's bailiwicks in terms of spells.
Divine Lorist: Relatively weak; notably, it's only about a quarter of what the Knowledge Domain gets (two languages AND two knowledge-type skills, with the double proficiency bonus). But it seemed the most appropriate skill for them, and nothing else really popped out.
Will of the Gods: Another slightly weak ability; a low-level cleric might choose to use it daily, and it's the sort of spell that seldom gets memorized because of its situational use.
Pantheist: Where they really shine. They can use ANY domain's domain spells, at a resource premium (i.e. they have to spend a Channel Divinity to unlock a domain, PLUS the usual spell slots). If you need a blaster, they can channel the Light domain's fire spells, or Tempest's weather spells. If you need a jackleg druid, well, there's the Nature domain. Need a healer? Boom. Life Domain. At level 6, they can choose 2 extra domains a day, for a while, if you don't need Augury or Turning Undead.
Bonus Cantrips: Not a great feature, not a strong feature, but one that adds to versatility. Other domains add damage to attacks... either your wisdom bonus to cantrips, or a die to weapons. This... does neither. You are a more clericy cleric than other clerics.
Friend of the Gods: At level 17, being able to ask for divine intervention 5 times per week (assuming a 20 Wisdom, which I think is fair with 3 ASIs behind you) is pretty neat, BUT it only really amounts to 5 bonus slots per week, with a chance of failure for each request. When you get to level 20, of course, it's a guaranteed 5 bonus slots per week but, well, you're level 20. You're one of the most powerful priests in the world, and you've spent a lot of experience points growing closer to ALL the gods, even ones who hate you. I don't have a big problem with "Can cast slightly more spells per week", especially when you don't actually get to pick the spells, the DM does.
It's a character designed to be a very clericy cleric, without being any specific type of cleric, while also being every type of cleric... just not as good as a specific type of cleric at being that type of cleric.