World of Warcraft:
Legion is going to launch some time in the middle of the year - my guess is mid-to-late June, so at that point we will have been on the Broken Isles for a few months. At most I think we might be getting ready for whatever Legion's second raiding tier will be. Assuming that they're continuing with the Warlords model (as disastrous as that might be, it's possible they started planning Legion before they realized what a bad idea a two-tier expansion is) that's probably going to be the Tomb of Sargeras, but I'm hoping that it will be a middle-tier - though given what we've seen of the Legion alpha, I don't know what that middle tier would entail.
Anyway, it's extremely unlikely we'll start hearing about the expansion that comes after Legion until 2017.
Starcraft just completed its Starcraft II trilogy. I honestly don't know what's in store for the series. The ending of Legacy of the Void really closed off the vast majority of the storyline. There's still room for conflict, certainly, and new characters might arise to carry forward, but I think after the long rollout of Starcraft II, we're probably going to see the series take a breather at the very least.
There was apparently some hiring call for people to work on a new Diablo game. The question then is whether it's a new expansion or Diablo IV. I'd wager more on the former, as Diablo III has gotten fairly popular, especially with the console version. A new class and a new act would work pretty well, and perhaps a bunch of new ideas brought into the seasonal system or the bounties/exploration mode. There's also a few story hooks that would carry directly from what's happened in DIII so far. I just hope they don't turn the Nephalem evil or something.
Hearthstone has been getting pretty regular periodic updates, so I don't expect to see anything terribly radical. Probably a new solo adventure and maybe a new expansion announced (by then, Whispers of the Old Gods will have been out for several months.)
Heroes of the Storm:
Like Hearthstone, Heroes is built for periodic small updates - throwing in a hero or map every now and then, and of course skins and mounts. I'm hoping we see more unusual maps like Towers of Doom (both the unique tile set and objectives.) Still, major shifts are probably going to be rare, so I'd simply look for these periodic update announcements.
Overwatch will have officially been out for a while by then. Given that it's a general-admission type game, we won't see individual new characters introduced. We might see a big chunk brought in as DLC, though. It'll be way too early to announced a full sequel though.
Blizzard has been expanding its franchises recently, so I wouldn't be shocked to hear them coming up with another major project to work on. Personally I hope we get something a little more solo/story-driven. As enjoyable as I'm finding Heroes of the Storm, I tend to prefer games with a solid solo campaign, and finding that "Project Titan" was scrapped in favor of making Overwatch a multiplayer-only shooter was pretty disappointing.