My players think 5E's exp advancement is way too fast.
They don't want to have a stable of characters or create a new character with every adventure path( or even do adventure paths for that matter). They want to play their characters for years and years through many story arcs and adventure lines.
I have experimented with exp advancement in our games(mostly be milestones and just having them leveling slower without tracking the exp) but it's hard because they really feel that the exp portion of the game is fun and like the constant accumulation of exp's.
So I came up with this idea.
Whenever a pc levels his exp total resets to 0. So from 1st to 2nd level is still just 300 exp but to make 3rd level once you make 2nd level you will need to make 900 more exp.
Basically this is around 6 X EXP of the book. Now it wouldn be 6 X slower because I would award exp also for story awards and social encounters and exploration as well as for treasure spent or whatever else i wanted to make more of a focus on our games.
Killing monsters would account for a much smaller portion of the exp's total than before.
So I think the advancement would be in the nature of 3X slower. Letting them have a much longer adventuring life while also giving them more of a portion of the games mechanics they seem to like.
To be honest though, I'm unsure of just how this would work out. It's one thing in theory to say"I wish my character would adventure for a lot longer" it's another to play the game and be fine with"I'm leveling so slowly".
I think eventually I might try it in some form. I do remember back in AD&D playing one character for years and it was fun, I'm just not sure it was MORE fun than playing in 5E style.
Well, that is kind of two edged sword here.
Many of us just love to play in DnD's sweet spot. That is levels from 6 to 10.
You are quite more powerfull than your common folk that you meet on the way, but still not in nation/world shattering league.
And staying here while enjoyable can get boring if new toys come very slowly, if at all.
Try the E6, E8 or E9 verstion with continous gestalt leveling.
Lets say that your leveling stops at level 9. great spot. +4 proficiency bonus. 1 level 5 spell for casters, level 3 spells for half casters. Some cool features for martial classes.
Now, when you get enough exp to "level" from that you gain 1st level in second class.
You do not gain any HP from leveling, no extra HD for healing. Unless the next class has bigger HD than your current then you exchange HPs and HDs for that level.
I.E. as 9th level cleric you have 8 HP + 8×5 HP, and 9 d8 HDs. if you gain a level of fighter as second Gestalt class, your 1st level HP goes from 8 to 10 and you exchange one of your d8 HDs with one d10 HD.
You gain all weapon/armor proficiencies as well as all special features of the second class.
No matter how many of this secondary levels you gain you never gain HP(except if you go for bigger HD then you just gain the difference), never gain higher proficiency bonus, or higher level spells.
This will make your character little more powerfull as you will have more options, but you are still limited by 9th level proficiency bonus, 9th level HPs, and 9th level class features.
And mostly still limited by 1 action, 1 bonus action and 1 reaction per turn.
You will feel of character growth in broadness of skill but not height of high level mostly campaign breaking powers.