I can't remember that we got to know much about Zenos past 'in' the game. And for me the 'bad childhood' argument is too weak. It's the standard villain. Bad because of bad. In my personal opinion Zenos was a very boring whining character. And I don't get all the fans of this mad murderer.

If they bring him back, this would be really frustrating to me. But hey, it's not about me! I am afraid it's more about fanservice and people who decided to like this murderer. xD

If I could choose, I would like to see Papalymo back. He was a good and funny guy and belongs to the story since 1.0. and the Gridania starters from this time lost a lot.
Papalymo, and somehow also Yda.
I have still hope! I know it's silly, but I still have hope to see him again. : )