Pokemon was never revealed before at E3 don't know why it would be there now especially since Nintendo isn't going all out this year...
Don't get your hopes up, better off waiting for news from corocoro
Because it's a primarily Western show and the games are usually 6-12 months behind the Japanese launches. This time they have a simultaneous launch worldwide and have expressed interest in making sure news is delivered to everyone at the same time, not just to the Japanese fanbase while we all wait and twiddle our thumbs. The only reason Pokemon.com hasn't already updated with the new Pokemon and features is because we're still going off leaks.
IGN have already confirmed Pokemon X/Y will have a precence at E3. Whether that's demo units, new announcements, a trailer or whatever else is yet to be seen, but it will be there. --- rawr