If they're gonna announce anything really big they're gonna announce it in the opening ceremony... gives the event more purpose, people will be talking about it all weekend, the press who is there will be covering the **** out of it all weekend, etc.
Dev Panels are for discussing technical aspects of already made games (IE asking about updates, this that and the other thing). It could be possible one of those Dev Panels is a dedicated QA session for whatever game they're are going to announce in 14 hours or so.
I'd think with all this hype though that it would be the first thing they'd say, and the press conference would be to discuss it...but that's just personal speculation. Apparently in the past they have announced their new games during press conferences, and this years is after the main event which ends at 1:00pm Paris Time.