Blizzard's mostly annual proprietary convention is coming next month, and they have just released the schedule. Of course, these schedules can sometimes give you a good clue to where their various games are going and if there's something exciting to anticipate.
There are several "What's Next" panels, which frequently imply expansions, though I've learned to take that with a grain of salt after being very confident in a new Diablo III expansion for previous Blizzcons. Also, the World of Warcraft "What's Next" panel specifically refers to World of Warcraft: Legion, and given that the company has made it clear that they're going to focus on really ensuring that Legion has legs, I honestly think even hinting at the expansion to follow Legion would not be met by a good response - something that they're aware of.
So let's break it down by franchise.
Hearthstone gets updates at a pretty good rate and is a huge cash cow for Blizzard. We just got a new adventure, so I'd expect to hear about a new expansion this time around. No hints as to what that will be so far, as far as I know.
Heroes of the Storm:
Heroes just added new maps (which I haven't actually played yet) and new characters. Much like Hearthstone, though, additions to the game come at a pretty regular pace, rather than big bursts. So I'd expect them to announce a couple of new heroes and possibly a map, as well as new skins of course.
Diablo's 20th anniversary is coming up. Diablo III is also very much ready for a new expansion, or perhaps we're ready for Diablo IV. I don't see a ton of Diablo panels, though, so I'll remain skeptical.
Starcraft has a "Foundations for the Future" panel. Obviously the grand Starcraft II trilogy has come to a conclusion (and frankly, the plot really seems tied up in a definitive way) so I am curious about what their plan for the future of the franchise is. Granted, the original Starcraft persisted as a popular multiplayer game basically until Starcraft II came out (and may even still be played) so I would bet that Blizzard's focus is going to be on tending to the multiplayer side of things.
Like Heroes of the Storm and Hearthstone, Overwatch is a game that gets frequent updates, and so I wouldn't expect to see a huge thing like an expansion or sequel yet. Instead I'd expect to maybe hear about new game modes, maps, and characters that will filter in over time.
Legion just came out a month and a half ago, and they're also making it clear that they're going to stick with this expansion for the long haul (fingers crossed they succeed at that) and so the idea of hearing about the next expansion is basically a 0% probability. In fact, I'm not even sure that we're going to hear about the second raid tier, as I imagine that the Nighthold raid won't even be open yet at that point, given that we'll be getting Trial of Valor in between Emerald Nightmare and Nighthold. 7.1 will probably come out some time before the end of the year, but not necessarily before Blizzcon. Still, if they wanted to reassure players, announcing content to follow the release of Nighthold would be very welcome.