I see that Misha is comeing back the 16th but will he be back as Cass or Jimmie?

Castiel, as far as I know. Here is the official description:
“The Born-Again Identity”
MISHA COLLINS RETURNS! — Lucifer (guest star Mark Pellegrino) drives Sam (Jared Padalecki) to the breaking point and Sam ends up in a mental hospital. Desperate to save his brother, Dean (Jensen Ackles) calls every hunter he knows for help. One of the hunters gives him the name of a healer who turns out to have a very familiar face (guest star Misha Collins). Robert Singer directed the episode written by Sera Gamble (#717).
link

it has to be caz or at least i hope it is cause the leviathens aren't gonna go down easy

I think he will be Castiel.

didn't jimmy die already in the fourth season right after he begged Cas to take him instead of his daughter? I think it has to be Cas.