Staff of the Magi, but for lower levels
So, the Staff of the Magi is one of my favorite magic items, but the issue that it and a lot of other high-level magic items have is that... they're just too strong. You only ever see them in level 20 one-shots, and I think that's lame. Here, I propose a leveling system for extremely powerful magic items, where more of the item's abilities are unlocked as your character progresses.
Level 1
At Level 1, the Staff of the Magi functions as a magical quarterstaff and arcane focus that grants the ability to cast the following spells at-will: Mage Hand, Light, Arcane Lock, and Detect Magic.
Level 3
At Level 3, the Staff of the Magi gains 6 charges. It regains 1D4 expended charges daily at dawn. You can expend two of these charges to cast one of the following spells: Web, Knock, or Flaming Sphere.
Level 5
At Level 5, the amount of charges the staff has increases to 8, and you gain the ability to spend three charges to cast Dispel Magic. Additionally, the amount of charges regained by the staff every day at dawn increases to 1D4 + 2.
Level 7
At Level 7, the amount of charges the staff has increases to 12, and you gain the ability to spend four charges to cast Wall of Fire or Ice Storm. Additionally, the staff now grants a +1 bonus to attack and damage rolls when it is used as a quarterstaff.
Level 9
At Level 9, the amount of charges the staff has increases to 16, and you gain the ability to spend five charges to cast Passwall or Telekinesis. Additionally, the amount of charges regained by the staff every day at dawn increases to 1D6 + 2.
Level 10
At Level 10, the amount of charges the staff has increases to 18. Additionally, you gain a +1 bonus to spell attack rolls and to your spell save DC, which increases to a +2 at 17th level.
Level 11
At Level 11, the amount of charges the staff has increases to 22, and you can cast Protection From Evil and Good as though it were a cantrip.
Level 12
At Level 12, the amount of charges the staff has increases to 24, and the amount of charges it regains daily at dawn increases to 2D6 + 2. Additionally, you gain the ability to cast Enlarge/Reduce as though it were a cantrip.
Level 14
At Level 14, the amount of charges the staff has increases to 26, and you gain the ability to spend seven charges to cast one of the following spells: Plane Shift, Fireball at 7th level, Lightning Bolt at 7th level, or Conjure Elemental.
Level 16
At Level 16, the amount of charges the staff has increases to 36. Additionally, you gain access to the Spell Absorption and Retributive Strike abilities.
Level 17
At Level 17, the amount of charges the staff regains daily at dawn increases to 3D6 + 2.
Level 18
At Level 18, The amount of charges the staff has increases to 40 Additionally, if you expend the last charge from your staff, roll a D20. If you get a 20, the staff immediately regains 1D12 + 1 charges.
Level 19
At Level 19, the amount of charges the staff regains daily at dawn increases to 4D6 + 2.
Level 20
At Level 20, the amount of charges the staff has increases to 50.