Major casting spoiler that we all knew was coming anyways.
Ben Edlund has confirmed that Gabriel is coming back for season six although he is unsure as in what form (a reincarnation?) nor has fully worked out how they are going to accomplish the task.
Also, in a prior spoiler it has been said that the crew is introducing a new angel character that is possibly recurring with this character description: A bit rogue and an attractive charmer who appears slightly manic on the surface but carries a great sadness. The cover name they are giving this one is Balthasar.
The first things that come to mind with that description is Gabriel or future!Cas lol, but maybe that's just me.
Just as a reminder tomorrow (thurs.) @ 5:00 GMT the rewatch marathon of s3 continues with,
3.7 "Fresh Blood"
3.8 "A Very Supernatural Christmas"
3.9 "Malleus Maleficarum"