Of the three E3s that have had Cube titles on display the one game that wowed me the most was...
Pikmin.
Pikmin wowed me because it was totally unique. It didn't have any connection to a pre-existing franchises, it looked different than any other title, and it's gameplay was original enough that it was hard to classify it into a genre. Some people said it was an RTS but it sure as hell plays different than any RTS I've played. It also came out of nowhere. There were no rumours of its existence, no announcement, no showing at a previous show. It debuted at E3 2001 and was playable. And it was a good game too. One of my fondest memories of my Cube is playing Pikmin for the first time. Not only was it a blast but it also gave me this feeling of playing something new. No matter what the Cube was worth owning because of this unique gameplay experience.
That's what I want to see at E3 2004. No, not Pikmin 2 (though I'm really looking forward to that game) but something that gives me that same feeling that Pikmin did. Something that's unique, comes out of nowhere, and is appealing.