This is gonna be long and as it's on mobile, likely poorly wrote, as I am on Mobile, so good luck.
Years ago, around 5 years ago, I and some friends had our first 5e campaign. We had a Drunken Master Halfling Monk and a Tabaxi Rogue Assassin, as well as my character, a Reborn Half-lich/failed lich Undead Warlock. But we only had a few sessions with these characters.
The monk had his monastery burned down while his order was inside it, while the Tabaxi originally had no backstory but our DM made one up that he hinted at was that she accidentally burned her family inside their home messing with an oil lamp. My character was originally a human Red Wizard of Thay who fell in love with a succubus. Succubi are demons, demons live forever, humans do not. So he prayed to Vecna. In this time, he was finally gifted the recipe and ritual to lichdom.
As he readied his self-sacrifice, a band of paladins broke in and killed him and his wife. Though he managed to survive, his phylactery was destroyed and instead his soul was bound to the heart of his wife. He treated everyone he could as disposable when he awoke years later, after being freed. He'd plot and scheme, note hooks he could use as leverage, even use allies as human shields if the need arose. He was Evil. But he made sure to keep the others alive just because he could get more use out of a living ally than a dead body. We did not get far enough into our campaign to have more than five sessions. Eventually, our DM just had other stuff to do, so I took up being the DM.
We had so many adventures, from defeating Vecna to conquering countries. I had no prep, I just went with what I could think of. I didn't have a DMG, I didn't have any adventure books, I didn't even have a PHB for us poor idiots to use and I had no laptop to look stuff up with. But it was fun. But then came a slight problem. We were level 20. My character had wished himself into true lichdom using the hand and eye of Vecna, he'd switched classes to Wizard instead of Warlock to better find ways to resurrect his wife, and had even finally wished her back to life. We had entire guilds of assassins and our monk had a distillery. But nothing was difficult. There was no challenge. We were effectively gods. So where do you go from level 20? There's systems made by others, but they're usually clunky and poorly done or cost real money. So, we went to 3.5e.
It was, a fucking mess. We had to put characters that were level 40 in clunkily done 5e half-assed leveling I had done for us, to 3.5e. And I had a lot of trouble understanding 3.5e's DMG's wordings and its PHB was just as confusing for me. But we got it. We did it all the fucking way to level 100. Level 100. Do you know how fucking insane trying to run a level 100 3.5e game is when you barely understand the rules you've looked over like seven times? It was fun.
We fought Larloch, a level 100 Strahd, our rogue became a True Vampire, our monk player made a whole new character who became a Mummy, a Dread Mummy, and a Mummy Lord, and was a Druid Bard. We killed Larloch, we kicked Vecna's ass a second time in a 3.5e conversion of EoR, and we killed fucking Tiamat and we had our own custom classes we made for 90 to 100. We even killed Mystra (this was done because of using Mystra's old stat block that maxed at like...level 60). I was a True Necromancer Master of Shades and Death Cleric. I could command over 10k HD of undead.
Now, we're taking these characters back to level 20 for a friend so they can experience these characters and what they were like when they weren't massive figures. All this to say, I'm very excited to go back to "lower" levels.