Thou hast heard, see all this; rather, thou didst hear, (now) see it all, i.e. see all the prophecies now fulfilled, which thou heardest in days gone by. Will ye not declare it? Will ye not for very shame make known generally the accordance between the prophecies and the events, which you cannot fail to see?
Will ye not become "my witnesses" (Isaiah 43:10), and turn away from your idols? I have showed thee; rather, I show thee; i.e. "I am about to show thee from this time new things, even hidden things, which thou knowest not"—things belonging to the new cycle of prophecy, not previously announced, but reserved for the present crisis (see the comment on Isaiah 48:3).
On the whole, the language used seems most consonant with the view of Dr. Kay, that the "new things" are those about to be revealed in the next section of the prophecy (Isaiah 49-53), things belonging to the coming of Christ, and the "new creation" which it will be the great object of his coming to bring about.