I'd focus on flexibility, and reactive abilities. The everyman fantasy is mostly about surviving when you shouldn't, paper with having narrative events compare to give you a chance to succeed/grow.
I'd want to avoid giving this class traditionally structured abilities, instead letting them make choices at the last minute. Things like letting them pick skill proficiencies mid encounter, pools of points you can spend for extra attacks, extra damage, rerolls, and so on.
I think the big marquee feature I'd lean in on is modified multiclassing, maybe even forced multiclassing, topping the class out at level 5. I think I'd do something like "when you take a level in a different class, you may exchange a level of Adventurer for another level in that class," so by 10th level, you'll have covered entirely to a "trained" class to represent your growth/learning.