Macbeth’s letter to his wife gives details of the appearance of the Weird Sisters and the strange prophesies which they reveal to Macbeth and Banquo. Macbeth describes how baffled he was when the witches disappeared, and then how he was further astonished when messengers arrived to tell him that he had been made Thane of Cawdor. This was the second greeting that the witches used, and the first of which was not true at the time of their salutation.
He is sharing this good news with his wife, as well as saying that their third statement was that he will be King, and he wishes to tell Lady Macbeth, as his ‘dearest partner of greatness’, their good fortune. He says that she will be rewarded, just as he is –
This have I thought good to deliver thee,
my dearest partner of greatness, that thou mightst not lose
the dues of rejoicing, by being ignorant of what greatness is
The suggestions which you give in your answer are not detailed in the letter. He may have shared his excitement at the glory of winning the battle, and begun to speculate on the demise of the king. At the early stages he thinks that he can become king without direct action:
If chance will have me king, why, chanc
may crown me
Without my stir.
I imagine at the time of the composition of the letter he sees the king, possibly Banquo, and Malcom as 'obstacles' and may not have formulated plans for murder as such. He may believe that another action would remove these restrictions from his way, and that further actions by teh witches would suffice.