I've never noticed before that in Isaiah 40:9 it is Zion/ Jerusalem that is the herald of the good news of the end of the exile.
I think this paves the way for the church on earth, not just as gathered in heaven, being considered 'Zion', since it is the church as gathered on earth that proclaims the good news of Jesus' exile-ending cross and resurrection.
It seems to me that this might help a little bit with the apparent 'tension' between the OT and NT perspectives on mission. Isaiah 2:1-4 views mission as the word going out from Jerusalem and bringing people in to Jerusalem. However, the NT sees the word going out from Jerusalem, sure, but it seems to stays out, creating localised congregations everywhere it goes. Where is the vision of the nations being brought into Jerusalem? Answers include
1. Jesus 'is' Jerusalem - the word draws people to him.
2. The new Jerusalem 'is' Jerusalem, the word draws people to this heavenly city.
To which I can now joyfully add a third
3. The Church on earth 'is' Jerusalem. The word goes out from her and draws people to her.
I don't think this says anything new at all. But I like this way of thinking about it since it helps us to see that mission work isn't just something that takes effect in the heavenlies, but also something that is manifested on the earth. We really do gather people to Zion, the community gathered around Jesus on earth (as it is in heaven). We might even say that part of experiencing salvation is experiencing the church as she is now on earth.
Which tells us something we all knew all along anyway - church-planting is part of the fufillment of Isaiah 2. Great.