How mysterious! A tweet from the official Final Fantasy XI account read, "The heck is this... Is something starting up...?!" This image was included in the tweet.
Back in 2011, former Final Fantasy XI Hiromichi Tanaka mentioned that Square Enix was considering a straight port of FFXI to the PS Vita. But at that time, it was undecided and there was no concrete plan to do the port.
So maybe this tweet is referring to a PS Vita version of Final Fantasy XI? Or maybe it's a Wii U version? Who knows! Square Enix does, that's who.