It becomes increasingly clear that YHWH's ways and thoughts are unlike Israel's in the same way that the skies overtop the earth. We cannot know finally what YHWH has in mind, because YHWH's mind is not ours at all! And that appears to be what Isaiah is driving at: if you think you know what YHWH is about, you are flat wrong. You don't!
This is one of Isaiah's major points from the beginning of his oracles. Isaiah 40:12-31 is a long paean to the fact that YHWH is greater, more mysterious, more amazing, more hidden than we can imagine, and that in YHWH's presence we are "grasshoppers and stubble" (Is. 40:22, 24). Isaiah calls his people and calls us to something we are loathe to take on: humility. How often we think we know what God wants down to the last detail, because we imagine that we have a red phone to the Almighty. I need to repent of the myriad times I have given the impression that I know just what God is up to as well as just whom God has chosen or not chosen to speak on God's behalf. Who made me God for a day? Not to mention you.
Let me suggest a different title for Isaiah 55: "An Invitation to Humility in the Presence of God." I think Isaiah would approve.