Probably, players can create their own content for the high tiers. Start a wiki-thread here at the ENWorld forum. A number of players can collaborate on it. When it starts to shape up and looks publishable, move it on over to the DMsGuild.
Really, official support probably drops off around level 13 and higher. When creating content, it helps to think in terms of ‘tiers’ to set the scope and genre.
Personally, I count the tiers as follows.
Level: Tier
0: Novice (aka Jack, Commoner)
1-4: Student (aka Apprentice, Page) - recognizable to local community (eg. school)
5-8: Professional (aka Journeyer, Squire, Adventurer) - recognizable to town
9-12: Expert (aka Master, Knight) - recognizable to region
13-16: Leader (aka Grandmaster, Noble) - recognizable to nation
17-20: Legend - recognizable to world or plane
21-24: Epic - recognizable to multiverse
25-28: Mythic
Student, Professional, and Expert receives strong official support.
Leader and Legend tiers need more content from the player community.
Support for Epic and Mythic tiers is virtually nonexistent.