Class specific advancement

[Ettina]

What if, instead of leveling from xp or milestones, each class had some specific thing they had to do to level up? The warlock would do a quest for their patron, the fighter takes downtime to train, etc. What would be good thematic options for other classes?

[noob]

What if, instead of leveling from xp or milestones, each class had some specific thing they had to do to level up? The warlock would do a quest for their patron, the fighter takes downtime to train, etc. What would be good thematic options for other classes?

The problem as you can see is that levelling up as a warlock is a quest in itself and so will give extra screen-time for the warlock for no good reason.
In general if you want each class to need a different way to gain levels you should avoid making it create a screen time disparity.

[ThanatosZero]

If quests needs to be involved with the classes, I recommend class specific challenges, which brings the players out of their comfort zone.

One might need to undergo a very vivid dream/illusion of their deity (the PC doesn't know that), where their faith and judgement is tested. Of course, the PC might be lead to a place of conflict, where the classic paladin will be confronted with a dilemma.

A lawful rogue might need to solve a crime. The opposite requires to steal a very valueable object.

A evil cleric might need to capture countless sapient beings, only to sacrifice them on their deities altar.

A ranger may have to use guile tactics to defeat a orc horde.

A barbarian must win a contest of strenght or prove themselves in battle.

A wizard may help colleagues in their quests of knowledge, while also giving you help in return.

[SandyAndy]

I played a campaign where every level would involve us doing a session or two focusing on each member of the party "earning" their next level and then a few sessions of the main story and a side quest or two. That kept the levelling pretty slow but gave everyone a chance to shine while giving us a break from the save-the-world schtick. It definitely won't work for every party but you might try it.