Back when I was a senior in high-school, I adopted a freshman dork who got me to watch Fullmetal Alchemist: Brotherhood (if only to get him to talk about something other than Skyrim). I'm gonna call him Baby Gronk. He was a good kid and I was trying to show him how to be cool, so I invited him to my next D&D campaign. This was a mistake.
Baby Gronk was dead set on playing as Alphonse. I okayed this. Eberron was not out at this point, so I asked him to present me with the homebrew he wants to use. We then had a little talk about how to mechanically handle being a hollow suit of armor (which he wanted to use as portable cat storage!) and I thought I'd got a good read on what his character is going to be since we both have watched FMA:B. I also made sure he understood that D&D is not like Skyrim; it can be fun to break the game mechanics, but at the end of the day you are playing make-believe with a table of people who are
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
I'm not asking the most powerful, but your personal favorite. Mine is Bigby's Hand; it's hilarious and I love it. (And since Mage Hand isn't concentration, you can have them both active at the same time and make them high-five!)