Luffy:
Why do I feel like JBR didn't die. HSJ told him that he had made plans so that both of them could escape the prison. So my theory is what if JBR didn't killed HSJ there and that was all his plan so that HSJ could leave the prison while everyone thinks he is dead. His prison guard friend was either fooled by him or either helped him out as JBR saved his fiance, so he simply repaid the favour. And in the end he simply acted out in front of OBY and KMC so that they believe he is on the deathbed and don't try to investigate much. I feel like JBR also faked his death to escape from the prison in the end too.
That's an interesting speculation.
Even though the chances are extremely low, there are particular scenes that could contribute to it.
So they explain that the brain transplant isn't really successful cause the mouse this was tested on, ending up dying.
Yet, there's a scene in the spin off "Mouse : predator" where they show that the dead mouse inside the box that was thrown away has actually dissapeared? Did it possibly regain consciousness?
It could be a hint at how Jang Bareum could actually survive.
Or maybe it's to hint that the surgery was actually successful therefore Oz will step in and keep continuing the mission. The post credit scene showed a surgeon conducting the same surgery on another patient. Could be related to Oz trying to transplant Jang bareum's brain to someone else's :0