At Level 20, Wizards get their Capstone Ability. Undeterred by metagaming a power build and resisting the in game temptation to sell your soul for two levels for some Eldritch Invocations you hit Pure Wizard level 20 and get Signature Spell. The miltiverse's most powerful spell caster. You can stand toe to toe with the forces created the universe and you get... extra third level spell slots?
I feel like this was a safety valve for balanced turned until the power stopped dribbling out at all. Choosing a Signature Spell for a Wizard, or any spell caster, is something that happens naturally through the story telling of the game. Having this mechanical moment to celebrate it should both happen much earlier and have a bigger impact.
Presented without play testing, an alternative.
Signature Spell
Starting at level 5, choose a Wizard spell in your spell book of 3rd level or lower to become your Signature Spell. You may cast your Signature Spell at its lowest level once per long rest without expending a spell slot. At Level 9, you may cast your Signature Spell at its lowest level three times per long rest without expending a spell slot. At level 13, you may cast your Signature Spell at its lowest level at will.
I would replace the Capstone with something like Spell Mastery (from level 18) extending to Spells up to 5th level or something.