Umm we probably will never know until we play the DLC.. This DLC isn't centered around going out to solstheim to kill this dragonborn.. There is always a reason for it.. Maybe he is plotting something or maybe he stole something we value.. A unanswerable question..
If you mean items.. Then his outfit probably.. Dragonshouts of his also..
I was thinking more along the lines of unique dragon shouts, permanent enhancements to the player character for having absorbed the first dovahkiin. Things that were actually kind of cool basically.