Some Magic Items possess sentience and Personality. Such an item might be possessed, Haunted by the spirit of a previous owner, or self--aware thanks to the magic used to create it. In any case, the item behaves like a character, complete with PersonalityQuirks, Ideals, bonds, and sometimes flaws. A sentient item might be a cherished ally to its wielder or a continual thorn in the side.

Considering that the basic property says 'Reroll if the moonblade already has a +3 bonus', I'd read that as meaning that +3 is the max. There is no weapon in 5e with a + higher than that, although they do say you can go higher if you really want to. I'd personally cap it at +3.

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Help with Moonblade stats 5e Discussion. Don't use the moonblade table, use the Artifact weapons table in the custom items section of the DMG. Also, don't be afraid to make it supremely powerful, tier 4 characters are on the cusp of being demigods. Extended crit-range, extra damage, giving the character wings, add spell effects etc, all.

Most sentient items are Weapons. Other kinds of items can manifest sentience, but consumable items such as Potions and Scrolls are never sentient.
Sentient Magic Items function as NPCs under the GM’s control. Any activated property of the item is under the item’s control, not its wielder’s. As long as the wielder maintains a good relationship with the item, the wielder can access those Properties normally. If the relationship is strained, the item can suppress its activated Properties or even turn them against the wielder.

Creating Sentient Magic Items

When you decide to make a magic item sentient, you create the item’s persona in the same way you would create an NPC, with a few exceptions described here.

Abilities

A sentient magic item has Intelligence, Wisdom, and Charisma scores. You can choose the item’s abilities or determine them randomly. To determine them randomly, roll 4d6 for each one, dropping the lowest roll and totaling the rest.
A sentient item has some ability to communicate, either by sharing its emotions, broadcasting its thoughts telepathically, or speaking aloud. You can choose how it communicates or roll on the following table.

d100
01 - 60	The item communicates by transmitting emotion to the creating carrying or wielding it.
61 - 90	The item can speak, read, and understand one or more Languages.
91 - 00	The item can speak, read, and understand one or more Languages. In addition, the item can communicate telepathically with any character that carries or wields it.

Senses

With sentience comes awareness. A sentient item can perceive its surroundings out to a limited range. You can choose its Senses or roll on the following table.

d4	Senses
1	Hearing and normal vision to 30 ft.
2	Hearing and normal vision to 60 ft.
3	Hearing and normal vision to 120 ft.
4	Hearing and Darkvision to 120 ft.

A sentient magic item has an Alignment. Its creator or Nature might suggest an Alignment. If not, you can pick an Alignment or roll on the following table.

d100
01 - 15	Lawful Good
16 - 35	Neutral Good
36 - 50	Chaotic Good
51 - 63	Lawful Neutral
64 - 73	Neutral
74 - 85	Chaotic Neutral
86 - 89	Lawful Evil
90 - 96	Neutral Evil
97 - 00	Chaotic Evil

You can give a sentient item an objective it pursues, perhaps to the exclusion of all else. As long as the wielder’s use of the item aligns with that SpecialPurpose, the item remains cooperative. Deviating from this course might cause conflict between the wielder and the item, and could even cause the item to prevent the use of its activated Properties. You can pick a SpecialPurpose or roll on the following table.

d10	Purpose
1	Aligned: The item seeks to defeat or destroy those of a diametrically opposed Alignment (such an item is never neutral).
2	Bane: The item seeks to defeat or destroy creates of a particular kind, such as Fiends, shapechangers, Trolls, or Wizards.
3	Protector: The item seeks to defend a particular race or kind of creature, such as Elves or Druids.
4	Crusader: The item seeks to defeat, weaken, or destroy the servants of a particular deity.
5	Templar: The item seeks to defend the servants and interests of a particular deity.
6	Destroyer: The item craves destruction and goads its user to fight arbitrarily.
7	Glory Seeker: The item seeks Renown as the greatest magic item in the world, by establishing its user as a famous or notorious figure.
8	Lore Seeker: The item craves knowledge or is determined to solve a Mystery, learn a Secret, or unravel a cryptic prophecy.
9	Destiny Seeker: The item is convinced that it and its wielder have key roles to play in future events.
10	Creator Seeker: The item seeks its creator and wants to understand why it was created.

Conflict

A sentient item has a will of its own, shaped by its Personality and Alignment. If its wielder acts in a manner opposed to the item’s Alignment or Purpose, conflict can arise. When such a conflict occurs, the item makes a Charisma check contested by the wielder’s Charisma check. If the item wins the contest, it makes one or more of the following demands:

The item insists on being carried or worn at all times.
The item demands that its wielder dispose of anything the item finds repugnant.
The item demands that its wielder pursue the item’s goals to the exclusion of all Other Goals.
The item demands to be given to someone else.

If its wielder refuses to comply with the item’s wishes, the item can do any or all of the following:

Make it impossible for its wielder to attune to it.
Suppress one or more of its activated Properties.
Attempt to take control of its wielder.

If a sentient item attempts to take control of its wielder, the wielder must make a Charisma Saving Throw, with a DC equal to 12 + the item’s Charisma modifier. On a failed save, the wielder is Charmed by the item for 1d12 hours. While Charmed, the wielder must try to follow the item’s commands. If the wielder takes damage, it can repeat the Saving Throw, Ending the Effect on a success. Whether the attempt to control its user succeeds or fails, the item can’t use this power again until the next dawn.
5e SRD >Gamemastering >Magic Items >

Contents

1 Magic Weapons & Ammunition

Adamantine Armor

Armor (medium or heavy, but not hide), uncommon

This suit of armor is reinforced with adamantine, one of the hardest substances in existence. While you’re wearing it, any critical hit against you becomes a normal hit.

Animated Shield

Armor (shield), very rare (requires attunement)

While holding this shield, you can speak its command word as a bonus action to cause it to animate. The shield leaps into the air and hovers in your space to protect you as if you were wielding it, leaving your hands free. The shield remains animated for 1 minute, until you use a bonus action to end this effect, or until you are incapacitated or die, at which point the shield falls to the ground or into your hand if you have one free.

Armor, +1, +2, or +3

Armor (light, medium, or heavy), rare (+1), very rare (+2), or legendary (+3)

You have a bonus to AC while wearing this armor. The bonus is determined by its rarity.

Armor of Invulnerability

Armor (plate), legendary (requires attunement)

You have resistance to nonmagical damage while you wear this armor.

Additionally, you can use an action to make yourself immune to nonmagical damage for 10 minutes or until you are no longer wearing the armor. Once this special action is used, it can’t be used again until the next dawn.

Moonblade 5e Dmg