Wii U games:
Super Mario and Mario Kart both for the Wii U will be shown to the public, and I hope there are playable demos available out there. I don't think Yarn Yoshi would be shown.
3DS games:
We could also see Mario Party 3DS that may be there as well, because the link below says the games to be released in Fall-Winter 2013 would be revealed during the event. We won't see Yoshi's Island 3DS, because it could come out next year. Not sure the same thing would happen to Mario & Luigi: Dream Team and Mario Golf: World Tour, but we'll see what happens...
Also, it's nice to see that SuperZambezi gets to see Vipsoccermaster in E3 inside Los Angeles. It's just awesome, I would visit E3 one day...
http://www.nintendolife.com/news/2013/05/nintendo_confirms_smash_bros_3d_mario_and_mario_kart_for_e3_nintendo_direct