MMO is in decline these days. The investment is too damn high for a risky market. There's no high profile mmo development anymore from the West. Only the Korean and Chinese left who are making mmo these days. The western devs are resorting to lower riks projects like moba, multiplayer games and such. Even if there's mmo, they're simplified like Destiny and Division. I believe there'll be no more AAA open world mmo from western developers for unforseeable future.
Yeah, Blizzard was, in a sense, kinda lucky that Titan ended up crumbling under its own weight and getting Overwatch from its ashes. The MMO landscape has changed quite a bit from when WoW was at its apex.
They'd be wise to not even try to make a WoW 2. It'd cause too many problems (split the resources, possibly kill WoW along with WoW 2 potentially flopping too. Kinda hard to capture lightning in a jar twice)
As to the topic at hand, I wouldn't mind seeing some sort of remaster of the original Warcraft trilogy. Don't change the lore or anything, just spruce it up a bit, rerelease them.
I also don't really see a Warcraft 4 happening anytime soon, though it could be set up in the past and explore things like the War of the Ancients, maybe.