Lots of thoughts, but my main one is disappointment. Brandon is well known for his twist endings that, once revealed, are incredibly fun to go back and look at with all of the foreshadowing pieces in place. There's always a little "tell" that something isn't what it appears. At least, normally.
Calamity's nature wasn't foreshadowed at all. Nothing to suggest that he's an angel, or a god in human form, or an old one, or whatever. This feels like an incredible letdown, and more like a setup for a series about superheroes than anything else. The initial premise for the Reckoners is well documented, but now that story is told. The next story is "what was Calamity?" but there aren't enough threads left open to really justify this question. I guess his race could be the next threat, but even then, why make it so blatant but not leave more teases? Maybe its just writing in YA with "there's always another secret" that feels so unfulfilling.
Honestly, this is the first Sanderson series I'm hesitant to return to (should the Reckoners return). Again, it might just be the nature of a series designed with a YA audience in mind, but this just doesn't feel complete, or even incomplete in a positive way.