Yeah, it’s probably Death. The only other powerful creature it could be might be Gabriel, but Death makes more sense. Especially since he could send Castiel back to life.

That, and it would account for the “not all of Cas being back” thing… so my bets are on human!Cas in the later season. Which, yes.
And I know there’s a butt load of “Gabe’s not dead” stuff out there but now given that Lucifer was stabbed the same way and didn’t die… well, it’s possible.

I keep seeing people talking about season 13 speculation about the nephilim and I was just wondering what your theory was?

My theory is that the nephilim were corrupted by Lucifer and became Princes of Hell. I’m pretty much basing this off the fact that Azazel and Jack have the same color irises.  

In terms of what’s going to happen to Jack, I personally think he’s not going to be 100% bad and it might be up to Sam to relate to him and help him defy the destiny that Lucifer seems to think he should fulfill. ‘Cause, you know, Sam had that whole boyking and vessel for Lucifer thing and if Lucifer is the new King of Hell, that technically means Jack’s next in line and a potential vessel for Lucifer (if a nephil can be possessed).