What's the difference between the sentences below in meaning :
Jack may be coming to see us tomorrow.
Jack may come to see us tomorrow.
1: Jack may be coming to see us tomorrow
implies that even though the speaker doesn't know whether Jack will come or not, that matter has already been decided, to at least some extent (possibly because Jack himself has already decided to come, we don't know the details).
2: Jack may come to see us tomorrow
is completely neutral as regards whether or not the die is cast (maybe nobody knows if he'll come).