By the way, I was just looking and the german site actually says
Secret Bonus Level
Perhaps it has something to do with Final Fantasy XIV?
I think the word "event" was just poor translation, so this opens up the doors to anything, it's not neccissarily just an event like a competition or something.
And remember, whatever it is isn't going to overshadow the FFXI anouncement, so it all depends really.
If they anounce something dull like Avatars, it'll likely be just a trailer or something
If they anounce an expansion, then it could be anything really, I mean this is where they can get people thinking "OH WOW I really want to play that, but it's gonna take soooo long! Oh I know I'll play FFXI and it's new expansion!"
Either that, or the two anouncements could be linked; crazy speculation begins here.(Note you should probably disregard most of this because it is just me dreaming)
-Cross game Linkshell chat: They were keeping hushhush about guilds, but why?
-Character transfers: The idea of keeping your name/Appearance/etc.
-A Lobby type area where the two worlds meet? You will have scaled back XI gra;hics though, of course.
-Perhaps we will be told XIV is set some thousands of years either before or ahead of XI, and we get to see how Vana'diel became Eorzia -- or vise versa
Could be anything...