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 No problem.

The class levels things was in order to allow class progression and also to make it so people who have been in a class for so long are more proficient at the job. But the main point is so that adventurers can advance in different classes and unlock higher tier classes. Like Paladin like I described or Fighter+Wiz=Mage Knight or something like that, there's tons of potential things you could do and material to draw from for this.

Also like you said it depends on how you make it. The main thing that appeals to me for it is so that you can create actual party compositions (Tank/Healer/DPS) as well as have people that perform different functions (like the Scout allowing further distance from the guilds) and then have them partied up in order to allow adventurers to be specialized. 

Having adventurers earn classes based on what they tend to do the most could be interesting but without having direct control/influence I could see this becoming problematic because most adventurers will tend to become the same thing (IE: No one would wind up a wizard because they are always moving around and fighting/having to return to places to heal/deposit gold and that sort of thing.) Ultimately like you said it all depends on how you want to do it because it's your game but yea just some suggestions! I love party management games and there's basically no/little idle games that do it well so here's hoping you create something fun to play!