Zelda - trailer, new demos, etc, etc. I don't expect a release date till e3.
Wii Relax - They were going to reveal it at e3, but decided to wait for an event that was more low-key. What could be more low key than GDC? I hope it gets a quick release date.
3DS - Obviously this will be shown. I hope they reveal the full launch line-up details including exact dates within the launch window.
eShop - Hope they reveal 3DSWare games, 3d Classics, GB Virtual Console, etc. I hope it is called 'eShop' because it is coming to Wii and DS as well (except for the 3d content of course).
---
The above is the least of what I think we'll get. Usually Nintendo doesn't do that much for GDC, so I don't expect any more than that. But, if I had a choice in the matter, I would want to see the following:
New Kirby Wii
New Kirby DS
Last Story
Xenoblade
Rhythm Heaven Wii
Pikmin 3
etc, etc, etc.
