Okay, if we are speculating, I want to present you my thoughts about Diablo's future:
Consider they will release something at Blizz'Con 18 for Diablo, we have some options:
Diablo 2 Remaster / Remake: In my opinion, this is not the big thing they are working on. I think this would be mostly for people who have played the Diablo 2 original, so they would not earn that much with Diablo 2 Remaster / Remake.
Diablo 4 (or however it is called): We know for sure, that they are working on an another Diablo game with a new engine. Sure it will be released in the future, but I think it will take a few more years. Yes, they are working on that project for more than a year, but consider it is a new game and Blizzard has the claim to be the best and so they will take the time to make a perfect game (in their eyes). Another thought make it unlikely to be released next Blizzcon: I've they start Diablo 4 while Diablo 3 is running well, Diablo will make itself a competitor. It would make much more sense if they cut down the support and Patches for Diablo 3 over the time, so the hunger for new things will increase and more Diablo 3 players will switch to Diablo 4. I hope the fact, that nothing for Diablo is released this BlizzCon isn't the first hint for that.
Something for Diablo 3: A new class DLC, or a whole new Addon.
Class DLC for Diablo 3: We know that the Necromancer was about 1 year in the construction before he was released. And if they will announce another class at the next Blizzcon they had about two years for the class. If they are working just on a class it seems to be more fitting if they have released it at that BlizzCon, one year after Necro. But they didn't.
Addon for Diablo 3: Ok, that is what I hope/think will happen. I said they are not only working on a class because if you remember the Addon Reaper of Souls, you mention that we got with that addon the crusader. So with the next addon, we will likely get another class, but I don't think it will be the druid. I think the druid will be released later as a Class-DLC like the Necromant. I see more clue with another class: the Amazons. But why? Look at Blizzards Heroes of Storm. The heroes can be separated by the games the takes place in, and if you look for the Diablo Heros you find two angels you find during Diablo 3 occasionally, (Tyrael and Auriel), some bosses you kill (Leoric, Asmodan, Malthael,...) and for every class a representative hero. In March 2016 a new Diablo Hero appeared: Xul, the Necromant, and a half year later the Blizzcon they announced the Necro. In April of 2017, a new Diablo Hero appeared: Cassia, an Amazon. With Reaper of Souls we also got a map, so we will likely get another map. I suggest the Skovos Islands. Westmark is connected to the crusaders, it is one of their big cities, so the new map would be connected to the class too. The Skovos Islands are the homes of the Amazons. In the town is Tyreal talking with horadim Lorath Nahr and one talk starts "Lorath, I haven't heard from the Horadrim we sent to the Skovos Iskes. - Skovos is a dangerous land. [...]". To see that in the context you have to look at the lore when Malthael is stealing the soulstone and Tyreal is sending the horadim to find the Nephalem, the heroes. One hint I still have. If you go to the upper cursed fate, which is implemented since 2.6.0, you find the labyrinth, which you cant find anywhere else in the Diablo world, and top of it you find a big symbol on the ground. Which is similar to the symbol of :
(http://vignette2.wikia.nocookie.net/diablo/images/8/8c/Kurast.png/revision/latest?cb=20150705061656) a concept art of Peter Lee for the Skovos Islands.
Okay, it was clear, that the new class wouldn't be released with the BlizzCon'17, it will come with the Addon, which will be announced at the BlizzCon'18. If that is true it will likely come about Mai 2019, that means if the started developing the Addon right after the Nekromant DLC they had about 2 years, which is the same timeframe between Diablo vanilla in 2012 and the Addon Reaper of Souls in 2014, but I don't think that the team has started developing the Addon right after Diablo vanilla was released.
So we will see it at BlizzCon18 wheater I am right, or not? No, we will see at GamesCon 2018, because I think they will release it at the GamesCon like they did it in 2013 with Reaper of Souls. (And Blizzard has a stand for strategy and roleplay at the GamesCon 2018)
In the current status, it is not possible to implement a PVP-Modus, because the damage is much too high, compared to the defense. Before they implement PVP, they have to rebalance all at first.
I would love to see something like "Tower Defence", similar the arena of trails (season 1-3).