I figure there's two ways to look at the ending. Either:
A) Quantum multi verse theory obliterates any sense of accomplishment as no matter how hard they try to break the circle there will always be an alternative universe with an different outcome. Even at the climax where Booker is drowned there would be an offshoot universe where he did not. It's an absolute, eternally.
or B) The themes of godliness and ascension are taken literally in the form of Elizabeth, who in the strictest sense becomes the god Comstock idolised himself as, and the Gods we speak of in religion. Just as the Lutece twins fragmented their existence across multiple universes, Elizabeth at her maximum power embodies the fabric of quantum space time, and is able to alter reality and time at will. She's literally god. And using her godliness, she breaks the circle by having Booker/Comstock drown at the source of the timeline divergence. And in turn destroys herself, essentially killing god along with her.
If both BioShock and Infinite dealt with characters with god complexes, trying to become god, the Infinite deals with the theme of literal godliness through Elizabeth. While Ryan and Comstock/Booker attempt to become god, Elizabeth is actually god. And in the end, just as they fall, so does she.
The post credits stuff muddies all of this though and leans more towards A.
I'm sure in some universe, as well as this one, we've spawned a torrent of fan art.