Add more celestials. There's only 2 that fit this spell, and that's ridiculous.

That spell has too low challenge rating even Summon Greater Demon which is level 4 spell has challenge rating 5 or level 6 spell Conjure Fey with challenge rating of 6 .

Two things: several of the other conjuration spells, such as the ones you mentioned,   have the risk that the summoned creature will turn on you if you lose concentration. Conjure Celestial dose not do that.

Second: Thematically speaking unlike  other summons, celestials typicaly are servants of divinity.  A deity wouldn't  allow just anyone to call forth their servants whenever they wanted, thus the high level required. 

With the release of Guide to  Ravnica, we now have two new options for this spell. The Felidar and Battleforce Angle. Of these two cr 5 celestial, the angle is the best option. 

With Ravnica, there is actually a total of 5 options. 

Monster Manual: Pegasus, Couatl, Unicorn

Guildmaster guide to Ravnica: Felidar, Battleorce Angel

Compared to the 31 fiends that are 5th level or lower. Adding 3 still doesn't give me many options.

wish there was a stronger version, even if it was a 9th level spell, cuz I would so use it just to be able to summon a ki-rin, which sadly is cr12

I'm playing a tempest cleric of the devourer. I doubt the devourer is chillin with couatls and unicorns.

This spell should include summons for other alignments besides good (fey, fiend, etc). Too many cleric spells just assume that the cleric is good aligned, doesnt make sense considering all the available domains and deities.

Probably the worst summons spell available as written, especially for the slot level required.

YESSS Couatls everywhere

https://www.dndbeyond.com/monsters/couatl

Quote from Bugrit >>

I'm playing a tempest cleric of the devourer. I doubt the devourer is chillin with couatls and unicorns.

This spell should include summons for other alignments besides good (fey, fiend, etc). Too many cleric spells just assume that the cleric is good aligned, doesnt make sense considering all the available domains and deities.

Probably the worst summons spell available as written, especially for the slot level required.

 The key difference is that the being summoned by this spell won't turn on and attack the summoner, unlike losing control with Conjure Elemental, Conjure Fey, Summon Lesser Demons (they attack nearest non-demon creatures which can include the caster), Summon Greater Demon (which can break free on its own) or Infernal Calling (which has a lot of hoops you need to jump through to get it to work as desired)

Quote from OverlordofMidgard >>

With Ravnica, there is actually a total of 5 options. 

Monster Manual: Pegasus, Couatl, Unicorn

Guildmaster guide to Ravnica: Felidar, Battleorce Angel

Pegasus, Couatl, Unicorn, and Hollyphant are the only creatures that fit that aren't homebrewed.

Felidar and Battleforce Angel are both too high CR, they can't be summoned by this. Unless you're really gonna use a level 9 slot to upcast this underwhelming spell.

Most of the other summons have a chance for the creature to,turn on you (and Coualtls are crazily strong for their CR)

Underwhelming compared to what? Most other summoning spells have a chance to the creature to turn on you, this doesn’t.

The Gate spell is exactly for this.

Summon Greater Demon gives a chance to the demon to turn on you. Congratulations, you just summoned a demon to kill you.

I mean, it wouldn't be hard to house-rule some Celestials for the players to use with this spell.

Use the Bandit Captain 'monster', replace 'deception' with 'perception' in the skills portion of the handout, replace the saves with the saves of a Deva, give it the resistances/immunities of a Deva, Darkvision out to 120 feet and the telepathy ability, give it a 1/day Heal for 20 hit-points and a +2 radiant damage to the weapon attacks and bang, you've got a CR 3 to 3 1/2 Celestial that could be a former mortal willing to descend from their pilgrimage up the 7 mounting heavens of Celestia to lend a hand to worthy souls, or maybe take any other CR 1-3 monster and repeat the above process for something similar to whatever faith-system the Cleric has.

I mean, Couatl are fun to summon, especially if you're running a Eastern/Oriental themed campaign and/or caster. Literal guardian celestials of knowledge and healing, which serpents are in traditional chinese mythology, who could have taken mortal appearance to guide the player(s) as children and whoops, you just summoned your Master who is very interested to hear how you're doing and is more than happy to help you out.

so. okay, currently there are a total of 8 optional targets for this spell, and without Theros, or up casting it, you only get two options, one of which Paladin's can summon using Find Greater Steed. now, im sure the Coautl is great, but the fact is a lot of people are putting the idea that this is better, because the creature can't turn on you, and your right, the spell just ends, and the celestial vanishes. now, i dont really have a problem with that. what i have a problem with is this is that this is one of the only summoning spells clerics have option to, assumes that your deity has command of a bunch of Coautls, and then, everybody here forgets that Summon Celestial can make a possibly better creature at this level with an ac of 19-21, 70 hp, & 3 attacks which can do either 2d6+10 damage, or 1d10+10 damage and give another person 1d10 temp hp and have a cr of 5. does it cost 500 gp worth of gold reliquary? yes. Would i still up cast that spell rather then summoning a Coautl? Maybe.

Quote from HalfOrcHeavyMetal >>

I mean, it wouldn't be hard to house-rule some Celestials for the players to use with this spell.

Use the Bandit Captain 'monster', replace 'deception' with 'perception' in the skills portion of the handout, replace the saves with the saves of a Deva, give it the resistances/immunities of a Deva, Darkvision out to 120 feet and the telepathy ability, give it a 1/day Heal for 20 hit-points and a +2 radiant damage to the weapon attacks and bang, you've got a CR 3 to 3 1/2 Celestial that could be a former mortal willing to descend from their pilgrimage up the 7 mounting heavens of Celestia to lend a hand to worthy souls, or maybe take any other CR 1-3 monster and repeat the above process for something similar to whatever faith-system the Cleric has.

I mean, Couatl are fun to summon, especially if you're running a Eastern/Oriental themed campaign and/or caster. Literal guardian celestials of knowledge and healing, which serpents are in traditional chinese mythology, who could have taken mortal appearance to guide the player(s) as children and whoops, you just summoned your Master who is very interested to hear how you're doing and is more than happy to help you out.

 Couatl, or Quetzalcoatl to be more accurate (not to be mistaken for the dinosaur), are from Aztec mythology. But I guess you could go with that.

Quote from HalfElfMonk >>

Add more celestials. There's only 2 that fit this spell, and that's ridiculous.

 To be honest DND has a severe lack of celestials anyways, probably because parties don't tend to fight them much. It would be nice to have a couple more.

There's an error in the spell's name. It should be called Conjure Couatl, since that's all you'll probably be summoning with it anyway. :P