Metroid isn't what I would consider the "one more thing" title that would get everyone's attention... Other M was so hated on and the Prime series hardly tore up the sales charts...
I would have to disagree with that assessment. I remember working at EA during that particular E3, and when Other M came up as the "one more thing" my entire QA bay lost its ****. It also made numerous "most anticipated" lists in the lead-up to its release (most notably Gametrailers' Top 10 Most Anticipated Games of the Year). Sure, we didn't know then how bad the game would end up being, but I can tell you from personal experience that that game got people excited.
As for the Prime series, I don't know why Prime 3 never really caught on, but Prime 1 was a million seller on the GameCube, which was impressive for its time and the install base of the GameCube. If Nintendo announced a Prime 4 as their "One more thing", it would at least get people interested in the
possibility of buying a Wii U.
I figure at this point Nintendo can
really only end their show on the new Zelda game; the new presumed Xenoblade game; or a new Metroid game, if they want to end their show with a bang. Mario's pretty destined to be a start of the show presentation, so that just leaves those unless Nintendo has a new or resurrected IP in the works.
Or Nintendo could just do something like launch some Nintendo Land fireworks again. Because that worked out so well last time.