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.