They usually start work on a new Zelda game immediately after finishing the last one. If you look up info about the games, the exact same design team made PH and TP... which were only a year apart. Yeah, one of those was on the DS and wasn't as intensive as TP, but they were making that engine entirely from scratch.

I think Link's Crossbow Training was supposed to tease information about a larger release. I mean, didn't that game seem to come out of nowhere? Sure, it came packaged with a peripheral, but the new Zapper was really just a shell to apply to a controller that already worked... LCT was short, but it was all new. And they could have waited to release something so small (comparitively) on this new WiiWare service.

Eiji Aonuma (project director for the last four Zelda games) hasn't been noticed doing anything else. I bet they're going to surprise us. Here's a quote from a recent interview (July 2007); this is right after PH was released.:

"With this DS Zelda, I and the staff, our eyes were opened to the possibilities of the hardware... So not only are we planning to do the same thing for the Wii, but to continue to do so for the DS as well."

This was a year after TP was out, and he directly mentions plans for another Wii Zelda. (The question, by the way, was "With Phantom Hourglass done, what's the future of the Zelda series?")

I think Nintendo is going to surprise everyone by releasing something sooner than we all think. Maybe a trailer at E3?

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My timeline theory:
Same sword, different day.