Things we will never know (that, if anyone cares)
-Whatever happened to the original characters that didn't show up or weren't even mentioned in Reborn.
-The final destiny of Parkman, Hiro and the Haitian.
-The identity of the Twins' father.
-What happened to Emily's original boyfriend. He was written off the show like he never existed.
Etc, etc...
There were too many plot holes, but I'll blame that on time travel (cough-writers-cough).
He dies at the end.