i think in this world all he needs is a sword with spirit n thats it's. y should he even try to become quency,hollow or shinigami he already very powerful and it's not like becoming the above will give him tremendous boost.
so then if leo and shinobu wants to go to another world then they need to charge the medallion with space power, and the only zanpakuto that i know possessing space power is byakko of yoruichi but she could not control it. Will leo and shinobu be stuck in bleach for a long time?