Because it's a console launch year, I assume Japan will be keeping up with E3. Also, they do play more then animu waifu games, you guys. I doubt that Atlus is going to reveal P5, though I wouldn't be shocked if they did right after E3. Something big is obviously happening on the Final Fantasy front, and at least in the west, and I'd assume Japan, there is a big overlap in that crowd. I wouldn't want to be talking about my 2D cutscene cult hit PS3 swan song game while SE is over there teasing the glorious HD pretty boys and girls with zippers spectacle that people have been yelling about for the better part of the decade. Also, considering Arena, I wouldn't be shocked at a close to simultaneous release. Maybe a week, maybe a month. But with the P4 anime, P4A and Catherine doing so well, it's not a much of a risk. P5, whenever it releases, will sell.
I also don't think that Atlus has nothing new to show though. Maybe a new localization, maybe an HD collection (Digital Devil Saga?) but it isn't P5.