DMG p. 231-232 describes many epic boons for after level 20. Two of those are:
Boon of High Magic
You gain one 9th-level spell slot, provided that you already have one.
Boon of Spell Recall
You can cast any spell you know or have prepared without expending a spell slot. Once you do so, you cant use this boon again until you finish a long rest.
I'm not sure why would I pick High Magic over Spell Recall. Spell Recall lets you cast a spell you know of any level (in other words, it gives you a spell slot of any level). High Magic gives me a single level 9 slot. As far as I know, you cannot gain level 9 slots with short rests, so it seems like one of the boons is just a narrower version of the other.
What are advantages of the Boon of High Magic over the Boon of Spell Recall?