When a promise is made, we are wise to consider the past form of the one making the promise. Have they been true to their word, and is what they promised in keeping with what we have observed of their character.
When God promises (through Isaiah) that something new is coming, we can be assured by God’s consistent faithfulness to past promises made. Also, God has consistently done hitherto unimagined things among his people.
Making humanity central to the creation story was new. Calling Abraham out to an unknown land was new. Making a covenant with the people was new.
Isaiah indicates God is again about to do something beyond the imagining of the people. Yet we know a good deal of the character of our God. It may be new (and thus unknown), but we can be sure it will be good - better than we could imagine!