I loved the book as well, though I am hoping that Joel does end up becoming a Rithmatist. I also really loved the Reckoners series and I love how that all ended, but I feel like not only would we as an audience but Joel as a protagonist would be jipped if either Joel did not become a Rithmatist or he became one at the very end of the series and we never got to see him grow and develop as a Rithmatist. Personally I'm hoping for Joel to become the Rithmatist version of whatever Nalzahar is, possibly bonding a more powerful or more important shadow blaze.
And here is hoping we get at least a few more books in the series.