Wheel of Time does get pretty epic...about Level 12 or 13, I reckon.
		
		
	 
Casual teleportation all the time, travel back and forth to another plane, battlefield magic that would be at least 8th level spells in D&D...sounds significantly higher level than that.
	
		
	
	
		
		
			I wonder if any success has ever been had with a campaign that wasn't strictly linear - where the players had x different character sheets for each level of their character from 1 to x, and on any given session, the GM would run an adventure from any point along a possible timeline, that perhaps took a few sessions to reach resolution, and with the overall effect resembling a kind of mosaic of adventures or stories that perhaps had an interconnecting thread running through them.
		
		
	 
I haven't done the flashback part yet, but it is something I've thought of in case it arises.
Here are the first three things I ran in 5e, right after the LMoP:
After LMoP, we had a near TPK, because the party decided they wanted to go after the green dragon after she ambushed them and they drive her off. And she was prepared. One PC survived, and I had one or two brief "after the closing credits" scenes where the souls of the dead in their respective afterlife got the "hey, so and so is trying to resurrect you, do you want to return"?
Then we had a 20th level one-shot. (I think one of two people elected to make a brand new character rather than play a leveled up version of their character.) When it started, the returning characters had daggers crafted from a green dragon.
The adventure was just a short little thing to see what it was like. Their was maybe one or two speed bump encounters, and then they were fighting a kraken in its lair, with an aboleth sidekick or two. It was fun! Just seeing the high level stuff in play in virtual isolation was neat.
The next was our "Savage Kings" theme advdnture, and the premise makes it difficult to remember its actual level, though it was likely around 20. I started by telling the group the premise that they would be playing sovereigns of non-standard races, and I would need to customize their character sheets to take care of that, so they just needed to choose class and subclass, name, and some character concept sort of stuff, and I would assign them the race that best fit. The mystery was to allow them to think I was talking about playing orcs and gnolls and such.
When I started, it was clear something unusual was going on. Each PC got a prelude scene. The cleric ended up being Thane of the Frost People, and the initial scene involved where he was holding audience also included some flavor where one of his people carried in a cage with a little scrawny pointy eared thing in it that they think was called a "welf" or something.
The premise, that they got to see revealed in-character was that they were all giants who also included: Queen of the Fire People (fighter), Chief of the Hill Folk (barbarian), High Regent of the Storm People (bard) and Emperor of the Cloud People (can't recall the class). It was an adventure set tens of thousands of years ago inspired by the ancient dragons vs giants lore in the MM. Elves and dwarves were still young, and the world was ruled by giants (who called themselves the People) and dragons (the Wyrms).
I basically took MM statblocks, adjusted the ability scores a bit, decided about how many levels worth they were, and then added on character classes to get what felt around 20th level.
The adventure involved the kings meeting together and deciding they needed to deal with the fact that the Wyrms had discovered a Well of Creation and were going to harness its power. So they gathered their forces, and went to enlist the leader of the Stone People and his armies. They visited an arch-hag to get an artifact to help them, and then there was a massive clash of armies: all six giants with associated auxiliary creatures to help, versus armies of all 10 types of dragons with each of the 5e age categories. (The head dragon could cast 9th level spells.)
We used a modified version of the early mass battle play test that allowed for larger units, and ran a mass battle, which ended with a pyrric victory and then a brief time lapse description of the passing of ages, with mountains whethering away, etc. A little "closing credits" scene in that far future had some human children dancing around in a circle singing a nursery rhyme that referred in a vaguely remembered way to the battle they had just fought tens of thousands of years earlier in a time where there were still floating earthburgs in the sky and no humans. Although the mass battle got to dragging on, the adventurer overall was really fun! It ran 15 sessions.
The third was only 10th level (in hindsight it would have been fun to gain a level at some point): "Questing Knights" theme adventure. For this one, I wanted to see what a game would feel like with knights of various clases in 5e. We had a Devotion Paladin, a War Cleric, a Battle Master Fighter, and a green knight flavored Ranger. It was the most linear adventure I've ever run. The party started getting the quest from a king, and then went down the road near the castle one direction having encounters along the way, after which they turned around and went down the road the other way past the castle for the final adventures. At one point they found an enscription in a cave about "here lies Chief so and so if the Hill People", which was the PC from the previous advdnture. Took 10 sessions. It was also fun!
3e (my friend DMed these) we had like a 5 year Castlevania themed Ravenloft adventure that ran from 5-19. Then we did some other stuff, and came back and played the Castlevania 2 sequel at level 19 (Dracula's curse has prevented us from gaining that last level). Lasted months. It was really fun.
Other 3e was a fantasy world zombie apocalypse. We started at level 1, played for a while, the jumped to about level 5, then somewhere around 10, and then finally somewhere above 15. Not sure where it ended. Overall was around a year I think. Also enjoyable!
The only one that didn't quite pan out was when we tried to do a 3e Epic Level Handbook thing. The theme was that it was in a world coming to its final days. It wasn't going to be about saving the world. I'm not actually sure what the goal was. It took an absurd amount of time and math to make my advanced T'ien Lung great Wyrms character, and combat was gruelingly slow. The concepts were awesome, but it didn't take long before we sort of just didn't have the next session and never came back to it.
Most recently, we played Memories of Holdenshire for Level Up, and then advanced the characters to around 15 level I think for a short follow up adventure to get more experience with the system. It was fun, though we decided to stick with 2014 D&D (I don't regret my Level Up purchase though, definitely a great option for some people).
Only one of those involved actually playing characters all the way from 5th level.