Biggest announcement Nintendo could make at the upcoming E3

[Hephaestos]

I think they'll show a nintendo game for the balance board, that will revigorate whatever slowdown Wii fit will have had by then.

A large audience type of game (puzzle or brain thing)

Then they'll show Mario Baseball

Then they'll show Disaster

Then they'll have a big trailer for their big christmass game. (one of Kid Icarus, FZero, Donkey Kong or Starfox)