What would Lady Macbeth say in a letter to Macbeth in response to the letter Macbeth gave her? How would she feel about the witches' prophecy?
I believe that Lady Macbeth is excited about the witches' prophecy. If she were appalled by the prophecy and wanted nothing to do with it, she would not have done the entire "unsex me" scene where she is praying to be bold and cold to help Macbeth become king. Just after that sequence, Macbeth arrives to greet his wife, and she says the following to him:
Great Glamis! worthy Cawdor!
Greater than both, by the all-hail hereafter!
Thy letters have transported me beyond
This ignorant present, and I feel now
The future in the instant.
Macbeth's letter "transported" her beyond her "ignorant" present? She's not exactly a dumb pauper. She and Macbeth are wealthy rulers in their own right. They have gained favor with King Duncan, and are being granted additional titles. Yet, that apparently is meaningless to Lady Macbeth when the possibility of queen is within sight. Yes, I think she is quite excited.
Lady Macbeth's letter to Macbeth would betray none of that excitement though. She admits that Macbeth is probably too weak willed to take the throne for himself through murder.
. . . yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way
Her letter would be full of congratulations on not dying in the battle. She would express her pride in Macbeth for becoming a newly minted thane. She would encourage him to hurry home, etc. She most definitely would not express her budding plot to Macbeth via a letter, because it would give him too much time to think about it. She knows that to get Macbeth to do anything, she must corner and press his emotions without letting his logic take over.