On the software front I expect a strong line up of both casual and core games, Nintendo no doubt saw the error of their ways last generation. Not creating a destination for core gamers early on left the Wii out in the cold later on in the generation. Whereas power might have been a problem during Wii's life span, its successor won't have to worry about that for at least two years. Even then Sony and Microsoft aren't likely to come out with super expensive machines that will blow Nintendo's out of the water. I imagine Nintendo will tout vastly improved third party support and a much stronger online presence, bolstered by a surprise partner, as reasons for the hardcore gamer to return to the fold.
On the 3DS front we are likely to hear a lot more about the eShop, 3D video and movie support, as well as the next round of high profile titles. I expect to hear a ton of information about Super Mario 3DS and maybe even a 2D Metroid for the series 25th anniversary. The system will need a couple of tricks up its sleeve to compete with the NGP, as it has strong momentum coming into e3 going up against a system that hasn't met sales expectations.
On the Wii front I bet we will see a blow out on The Legend of Zelda: Skyward Sword information, as well as the U.S. announcement of the new Kirby game by HAL Laboratories. We might even see a couple of surprise localizations like Xenoblade and The Last Story. I wouldn't be surprised if Nintendo also detailed plans on how the Wii's successor would upscale these games to make them look really good on the new system. This is bound to be a really exciting e3 for Nintendo fans.