Xenosaga 4, 5, and 6 have been announced!!!
RPGamer - News Bulletin - Xenosaga IV: Die Geburt der Tragödie Announced

Also...
"Initial reports have been somewhat sketchy, but we do know that Episode IV is the precursor to Xenogears, which was dubbed "Episode V" in its credits."

They say that Xenogears is actually a part of Xenosaga?! Let me play all of the games! I should beat all of those games.