Nobody knows anything but here are my highly specific prediction. Uh huh.Anyone in this thread who says "This is the final tier" or "this is the final patch" or "this isn't the last one", none of these people know anything nor does anyone know but Blizzard. There could be two more tiers for all we know. Or this could be it, and the next expansion will be out by Summer 2018. You don't know.
Also, anyone who says "Lol the next expansion won't be out til 2019 at the earliest", were the same people who said 7.3 wouldn't even be on the PTR until after Blizzcon.
edit: I predicted 7.2 and 7.2.5's release schedule within 2 weeks~, months before it came out. I also predicted 7.3 would be up on the PTR by the end of this month, and it was.
My next prediction, 7.3 will be live on August 28th or Sept 4th.
Gamescom, which is during the last week of August, will have Blizzard showcase either the next patch(es), 7.3.5 and 7.4 - or reveal the next expansion. Probably both.
7.3.5, likely to be released just before Antorus raid gets released (the Argus raid), will probably open up just before Blizzcon, giving them plenty of opportunity to either showcase the previously mentioned 7.4 (if revealed at Gamescom) or the next expansion.
By Feb of 2018, we will likely have our 7.4, if that's something they want to do, perhaps a "bridge" between this expansion and the next.
Next expansion could come by Summer 2018, giving us roughly 5-6 months of 7.4, much of which would actually be content and so it wouldn't feel like a drought, and it would have given Legion a 2-year lifespan.
That's my prediction based entirely on their recent release schedules and other various clues in their Q&As and other information (keep in mind, they said Gamescom will be their best showing yet this year - which means that they will have to top last year's show, in which they revealed an expansion).
I'll caveat all of this with saying, once again, no one knows.