It's a full class.
It's John the Bakers Boy who follows behind Sir Gary, Reverend Sally, and Harzad the Great. Then later Johnny learns he is a descendant of Samuel Strong, Hero of the Second Empire and his incredible luck is the residual luck is a mixture of blessing of the gods and his strong bloodline causing near misses against him and heart strikes from him. Then when the 4th Empire is crown a new ruler, they all realize Johnny is one of the destined heroes of a prophecy anytimr the Empire is in danger
No months of special swordsmanship training.
No decades of arcane study
No hours of stealth missions
Just learning on the field and lucky hits.
First, Baker's Boy is a background. Second, the Zero-to-Hero trope works with ANY existing class. The only thing that it doesn't reflect is the character being worthless at level 1 compared to other level 1 peers.
But let's fiddle with the idea of a character that comes from a divine-favored bloodline of luck. Let's call it a "Scion". So as level 1 Scion, every "Scion" has to start their adventure has a henchman proficient only in Simple Weapons and no Armor training, no magic abilities or spells, and everyone else is better than them. And through henchman-work and field learning, they just magically become proficient, or even expert over time with just about anything the player wants (but not following the training of any other established class' structure), so the player can modularly recreate their vision of the character and the gear they evolve to use? Because hours, weeks, months, and years worth of time DOES pass in the campaign. They do have to become trained in gear, or they are stuck with simple weapons and no armor.
What kind of gear would three different L10 Scions use, and how would they fight in ways that would be different from each other and other classes?
Also how would "luck" and "survivability" be implemented in a way that does not overlap with the Lucky feat or the Halfling ability? Or Toughness? Do they have heroic advantage, or add extra dice, or double proficiency modifier, on all ability checks? Attack rolls? Saving throws? Damage rolls? Evasion-type abilities? Max Hit Points? Auto-success?
If not trickling out those types of generic bonuses over time, there doesn't seem to be enough generic design space for a class that is supposed to represent the idea of "A nobody with no training, that somehow gets training that has nothing to do with any other classes of the more skilled people he's traveling with and learning from."
I guess there is a market for players who want a zero-to-hero demigod who just succeeds at everything they attempt, and survives all the time. It might work for a Mary Sue story, but I'm not interested in that kind of design for a PC class. But I understand how others might be interested, so I wish them good luck and great fun!