So, I wrote a handful of high level adventures for TSR back in the day. 2e. The highest level spell I had to account for was 7th. My feedback was quite positive, although not uniformly so. Unfortunately, part of my direction was they were uniformly fetch quests.
The keys include:
The characters are required to use their potent abilities to achieve their goal. Commune, find the path, conjure elemental, wall of force- if you don't have these you fail. Information is minimal to absent.
Travel is non-standard. Not only is it far away, but you don't really know where you are going. Sure, you can teleport, but can you take everyone with you? What level of teleport failure are you willing to risk? Maybe you need to summon a guide to tell you where to teleport to. Can you teleport your whole party, mounts, hench, NPCs and all? You might need two wizards. Combats are mid-air or on floating islands, everyone had better be able to fly.
There is time pressure. Now, here, I actually don't mean you have 5 rounds, or 6 hours, or whatever to complete the entire thing. You may have 5 rounds to solve a particular encounter, but what I mean here is that you have enough time to visit five of the seven places you would like to go. You need to be at the first encounter, and the last, but of the remaining five you only get to visit three. Choose wisely; the party has a number of resources to determine what gives you the best chance to "win" the adventure. Importantly, not visiting one or two of the sites does not cause a loss! There can be levels of victory.
The climatic encounter is not necessarily a combat, or only combat. For one of my adventures I shamelessly stole from the Nordic myths. I took the scene where Thor and Loki visit the lodge of Utgard-Loki; the PCs had to have a boasting contest (OMG, they had so much fun with that), an eating contest with Fire, a drinking contest with the Sea, and a wrestling contest with Old Age. Not that the PCs knew they were interacting with those personifications.
The win-state is gated by role-playing. You have to convince Mimir to tell you where the Gilded Comb is, and what word puts the guardian dragon to sleep. (Or, you could fight it if you wanted. I wouldn't take that from you.)
There is at least one battle with peers. You have to give the combat monsters a chance to monsterize. It's why they show up, and they don't want it to be easy.
Missed one~~
They're not saving the world. Saving the town, city, metropolis, kingdom, people, the Zola Fel, fine. Not the world, because then they can't lose credibly. There always has to be more to protect if the adventure goes sideways.