I am still very new to the D&D System and need help creating an item. My DM asked me to create a legendary artefact for my lvl 3 wizard (school of conjuration) half-elf. The artefact shall grow stronger when I level up and have some legendary active and passive effects. We are playing a High-Fantasy Campaign. I already asked my DM for help, but he has no ideas for my char.

Thanks in advance

Well, items can't unlock new features over time, so you will have to edit the item to add new features.

What do you want the item to do?

What if it's like a staff that summons or turns into creatures, like a low-grade summon spell? Maybe you can borrow from the druid wild shape ability so it scales up as you level? 

Have abilities tied to your level:

Recover a number of spell levels in spell slots equal to half your level (i.e. level 6 caster can recover up to a lvl 3 spell slot or 3 lvl 1 spell slots). Ability recharges at dawn.

Melee attacks do additional damage equal to your Int bonus - minimum 1.

Spells can be upcast at one level higher in potency (normal spell slot cost - a lvl 1 spell can be cast at lvl 2 with only a lvl 1 spell slot). This ability can be used a number of times equal to half your level. Ability recharges at dawn.

You can expend spell slots to counter a spell cast at you. The cost is equal to the spell lvl used against you - Lightning Bolt would require a lvl 3 spell slot to counter.