If they add some manner of adoption mechanic, they might become playable.There was a few peasants that rose in the ranks to the level they would be represented as characters in CK3 but these was quite rare and since the game is about dynasties it would be kind of hard to play as a former peasant since you would need to start as that character.
As it is, the various random (non-noble) character adding methods, and bestowing of titles and positions cover the likes of Thomas Churchill or William Marshall.