Aside from what was already announced at last year's E3 that has yet to be released (Xenoblade, Yoshi, Mario Maker, Star Fox)
Things I think they will show:
For Nintendo 3DS:
Pokemon Z or X2/Y2.
Fire Emblem: IF
Yokai Watch localization details
One new IP
One New 3DS exclusive
For Wii U:
Animal Crossing or Metroid Wii U, one of those two.
Splatoon DLC Pack 1
Mario Kart 8 DLC Pack 3
New game Retro has been working on
Things I'm hoping for:
Nintendo 3DS:
Metroid 3D - A return to Samus' 2D roots would be appreciated.
Some key localizations (Dragon Quest VII, insert any other you want)
Wii U:
Paper Mario U
The Legend of Zelda: Twilight Princess HD
Hardware Comparison Threads:
Current Thread
Is Hardware Getting TOO Powerful?
Older Threads:
PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017