There's a lot of awesome stuff that could be at E3. Even if it is only 2015 games, which I highly doubt (they aren't NOT going to mention Zelda at all, or show us what is coming early 2016), the end of the year line up seems scarce.
By the time of E3, the only big games left for the year will be Star Fox (which will obviously be focused on a lot), Xenoblade X, and Fire Emblem If.
Everything else is smaller releases, like Mario Maker (which for all we know could come before E3), and maybe Wooly World (which is supposed to come before E3, but it may be delayed since we haven't seen it for a while).
And most of those games I said seem to be coming out during the summer, if not before.
Miyamoto said StarFox was planned to come before Zelda (which at the time was most likely planned for a late November release), Xenoblade X comes out NEXT MONTH IN JAPAN, and If is planned for a Q2/Q3 Release in Japan.
The Fall and Holiday Line-Up is really empty when you think about it.
There has to be new games, especially with Zelda not being there for the holidays.
Also, they are almost certainly going to talk about the further of Smash DLC.
I think we all know that, right?
EDIT: Also, they have to confirm Pokémon Z. We all know it's coming. That may even be announced before E3, as most Pokémon games are.
EDIT 2: Also, I'm seeing them confirm Pokken for Wii U, and give a release date. That could be the Big Holiday System-Seller.