I think the only thing we can say with confidence about poor Zathras is that nobody listened to him. /forums/images/icons/smile.gif
Actually, JMS has made reference to the adventures Valen and Zathras had in the 1250s ("quite a tale, actually"), which are outlined in his notes, but he's never been more specific, nor indicated that he has plans to tell us about these plans in any form. (Apart from pretty much ruling out the notion of a TV movie, both on practical grounds - only the hardest of hardcore B5 fans would be interested in a story with no Humans and hardly any non-Minbari - and because a fan posted a too-detailed story idea on a site where JMS was, then, still lurking.)
Maybe a novel or graphic novel will turn up one day.
Regards,
Joe