When do you think 9.0 will be released? I feel like with how late the 8.3 raid will be released and the sheer volume of bosses it has, that the next expansion won't be out for a while.
If they push out the 12 boss raid in January, that's already 2 months after Antorus. From that alone, that would put us in October.
Given that there will be other things to keep us occupied like the 15th anniversary, I wouldn't be surprised if Blizzard goes for a more traditional holiday release like November when people are back from their holidays. I don't think the summer release worked for either Legion or BFA.