I'm not sure that's the best guess. While it would be the obvious choice, I don't think much could be said to be "earthshaking" there, and Knuth isn't one to throw out hyperboles. I think it will be something new, almost certainly in a different field, and it will involve looking at an unsolved problem in a way different than it's previously been approached from, much like TeX. I can't wait to see what he announces.

But then, it is not announced as an earthshaking announcement, but as an "earthshaking announcement." The quotes make it sufficiently vague that it could be anything from "there was a typo in a comment in the metafont source", to "Vol. 4 is off to the printer."

Vol 4 is off to the printer. He's already announced that the first 5 fascicles will be appearing in hardcover later this year.

I'm willing to bet that it is not. The words are in quote marks which suggests nothing majority earth shattering (on the wider stage).

Probably a big TeX thing, given the context.

It's titled "A Special Announcement!" in the TUG conference proceeding. I'd say the odds are pretty good that it's TeX-related.