Once a spellcaster has reached 17th level and can cast 9th level spells, what more power can they try to obtain?
Before Mystra’s ban, there were 10th and higher level spells. Back in D&D3.5 there was Epic Magic. Now in 5th Edition, in lore, what spells or magic can the greatest wizards, sorcerers, or others seek to obtain?
Although it cannot be cast by players in this version, I suppose Epic Magic could still be a thing. As for elves, Elven High Magic would be a goal.
Best Answer
Epic Boons
There are no higher-level spells in 5e than 9th level for player characters (that I know of)1. More powerful magic is more of a worldbuilding and storytelling tool. Yes Mythals and Elven High Magic still exist in the world, but not as something that player characters have access to.
For NPCs, getting access to and control over such high magic sources is a common story motive. For example, in Dungeon of the Mad Mage, Halaster and several other high level spellcasters are vying for control over the mythical source of power in the dungeon.
For PCs, once characters reach 20th level, the DM can award epic boons instead of higher level spells. These will improve the characters' spellcasting ability in some way beyond that of "normal" casters.
These are certainly something to "strive for" as a player character. While they do not grant higher level spells, they enhance how well characters are able to use their existing spells. For what you are asking, the Boon of High Magic seems to be thematically the best fit:
Other Boons that are concerned with better spellcasting are the boon of spell mastery, boon of spell recall, or boon of quick casting (all on p. 232 DMG).
It might be a nice capstone for the end of a campaign if the DM grants the players control over one of the high magic resources in the world, and they then become NPCs shaping the world in further adventures for new groups of PCs. (We did that with many of our high level characters at the end of campaigns across various editions of D&D, it is a pretty satisfying experience for closing things out).
1 As Joel points out in his answer, you technically can cast a 9th level spell as if it were 10th level with the help of the Book of Exalted Deeds. While I think that's not what your asking for, it is a pretty cool exception.