Any possibility of an Onion Knight or 'Bare' class in the game was in 1.0's very early development where SE considered a basic 'Adventurer' class (abbreviation 'SPC') that was literal bare handed and presumedly could equip anything. But nothing ever came of it when 1.0 was eventually released.

As it is Onion gear is already in the game as equipment that any class (or in some circumstances, certain existing classes) can equip (Onion Helm was a bonus item included with the 1.0 Collector's Edition, Red Onion Helm and Peregrine Helm are very rare Venture possibilities, the rest of the Onion Set is also found this way, the Onion Shield for GLA/PLD that is bought with Tomes of Poetics, and then there is the seperate Onion set for LNC/DRG that is found in the Labyrinth of the Ancients), so it's therefore pretty clear that precludes the actual Onion Knight Job appearing in the game (lore wise Onion Knights were actually not a class in Eorzea but robots used by the Allagan Empire, so that's another nail in the coffin I'm afriad).