It's pretty surprising that GameFreak decided to actually have something dedicated to main series Pokemon games during this year's E3, but then again maybe it's only because the English versions are being released on the same month as Japan. Either way, it's going to be interesting.
Given that E3 is about showing off new features and the like in upcoming games, I don't think it is much of a stretch to think that we will be getting some new information. It also helps that June 11 is so close to the 15th, when Coro Coro releases and the English website updates, so they could very well be planning to share stuff that they were going to reveal very soon anyway (or vice versa; maybe Coro Coro and the website are being planned to update with E3's information).
Even if they give us new information that isn't about a hypothetical new type, we could very well be getting some information about what makes these games different in a mechanical sense. I have to wonder if they'll choose this presentation as the right time to elaborate on this very interesting quote from Masuda regarding the games from his blog:
"A new battle mechanism that allows your Pokémon to become even more powerful
when the bonds you share grow stronger."