Level 20 Loot

My party is coming up on the end of a 2yr campaign lvls 1-20!!
I want to load my next temple up with some holy grail items to beef the team up before the finale. Stuff every party wishes to wield one day.
For context, we have two barbarians, a bard, a cleric, and a monk.
I have a pile of stuff I've been sifting through, but anything you good folks could point out that I may have overlooked or not considered would be wonderful. I'm grateful for the assistance, and thanks in advance~

Greetings EmberFrog,

Sounds like you are looking for strong Magic Items to give to your players to make them stronger.

Outside of the standard Magic Items you most likely already looked through, have you thought about rewarding them with any of the following:

• Epic Boon Feats
• Super Natural Gifts
• Titles (not strength in game, but something to be proud of)

Cheers!

You could give them relics that allow them to take levels in another class, effectively leveling up beyond level 20.

The class info is helpful, but in this case, I’d really try to tailor it to the specific character, so I’d want more details before commenting. (e.g. an axe of the dwarvish lords is always cool, but if someone is a dwarf, it’s going to be extra cool.) I’d start with a magic item search, and filter it by artifact. 
And I’d definitely give at least one of them a sentient weapon. 

We once had home made magic items to end a campaign.

The 6 items of the lords. All sentient all want you to find all the others and bring them together. They constantly try to convince you to bring them all together.

(The secret part)
Then after finding and collecting them all you realize they just wanted to get together to argue with each other till they get so upset they all explode at once to shut the others up. Something close to a nuclear explosion. Which kills everyone (all the characters)around them and destroys the city they are in.
(double secret part)
The items don't get destroyed but just get thrown to places all around the world to start the whole thing over.

This could be why some cities around the world are in total ruin.

I've been digging around in the older editions for cool abilities. I've settled on taking useful items and imbuing them with the abilities of a prestige class. For example:

Beggar So’s Horn of Zui Quan (Zway Chwahn) - This hollow wild yak horn is mysteriously always full of jiu (jee-oh.) Engraved with silver filigree, "The body is drunk, but the mind is clear," the wielder of this tankard is now a drunken master.

Drink Like a Demon (Ex): A drunken master’s body handles alcohol differently from other people’s. They can drink a large tankard of ale, a bottle of wine, or a corresponding amount of stronger alcohol as a move action (half their movement speed.) Every bottle or tankard of alcohol they consume during combat reduces their Wisdom and Intelligence by 2 points each, but increases their Strength or Constitution (character’s choice) by 2 points. A drunken master may benefit from a number of drinks equal to their proficiency bonus. The duration of both the penalty and the bonus is a number of minutes equal to the drunken master’s level + proficiency bonus.
Stagger (Ex): Once per turn, by tripping, stumbling, and staggering, a drunken master can make a charge attack that surprises their opponents. This ability has two beneficial aspects: First, the charge need not be in a straight line. Second, their movement through threatened squares provokes no attacks of opportunity. Move 10+ feet toward a target and hit with a melee attack (as part of the Attack action) to deal an extra 1d8 damage or push the target 10 feet.
Corkscrew Rush (Ex): A drunken master can perform this maneuver, leaping forward and twisting their body in midair as they attempt to head-butt an opponent. When making a charge attack they can, in addition to dealing normal damage, spend a focus point to initiate a corkscrew rush. If the attempt succeeds, the opponent is stunned unless they make a Con save (DC 8 + proficiency bonus + Wis mod). However, if the attempt fails, the drunken master lands prone in front of the opponent.
Drunken Carp (Ex): A drunken master can lie prone with their feet ready to strike upward at an opponent. Spend a focus point to switch to this stance, where their melee attacks are no longer at disadvantage, opponents’ melee attacks are no longer at advantage, and ranged attacks are at disadvantage.
Swaying Waist (Ex): A drunken master knows how to weave and bob during an attack, making them more difficult to hit. The character gains a +1 bonus to Armor Class for each bottle or tankard consumed, up to their proficiency bonus.
Improved Feint (Ex): A drunken master can make a Deception check as a move action (half their movement speed) to gain advantage on an attack.
Improved Grapple (Ex): A drunken master gets +4 to Grapple Checks.
Superior Improvised Weapons (Ex): While bottles and tankards are a drunken master’s preferred improvised weapons, they can use furniture, farm implements, or nearly anything else at hand to attack their foes. A drunken master can use long improvised weapons (such as ladders) as reach weapons according to their length, and improvised weapons with many protrusions (such as chairs) provide a +2 bonus on opponents’ disarm attempts. Finally, large objects with broad, flat surfaces (such as tables) can be upended to become improvised tower shields. Furthermore, a drunken master wielding an improvised weapon deals an extra 1d12 points of damage.
For Medicinal Purposes (Sp): A drunken master gains the ability to convert a single alcoholic drink they have ingested into a single potion of greater healing (4d4+4,) as if they had just drank the potion. To use this ability, the character must be under the effect of an alcoholic drink (see Drink Like a Demon, above). When they convert one drink of alcohol into one dose of the potion, their ability scores change (+2 to Intelligence and Wisdom, –2 to Strength or Constitution) as if the duration of the alcohol’s effect had expired. This ability can be used up to three times per day. It is a bonus action that does not provoke an attack of opportunity.
Breath of Flame (Sp): A drunken master can ignite some of the alcohol within their body and spew it forth from their mouth as a free action. This breath of flame deals 3d12 points of fire damage to all within the 20-foot cone, or half damage to those who make a Dex save (DC 8 + proficiency bonus + Con mod.) Each time a drunken master uses breath of flame, it consumes one drink’s worth of alcohol from within their body, lessening the penalty to their Wisdom and Intelligence scores and reducing the bonus to their Strength or Constitution score (character’s choice.)

I decided that a character that uses this item, that doesn't already have focus (ki) points, gets 4 focus points. Same as the number of superiority dice a battle master starts with.

If you guys think of any abilities or prestige classes I should definitely look into, let me know. Thanks for the feedback!!