I'm surprised with all this speculation that no one has brought up the end of ROS. Specifically, where they talk about the future of the nephalem and how they will be "tempted to corruption" and could turn good or evil. At the time, I was assuming it meant a big PVP expansion, but now I'm leaning more toward mmo with two opposing factions. -shrug. None of us know, but its just interesting to theorize. Does anyone else feel this ending is key in the next installment?
They'd have to pull out all the stops to get me over to an mmo set in the diablo universe (over wow). imo they should keep it as an arpg to keep the appeal. (let's hope it's not a card game)
An MMO does not need to play like WoW, or anywhere close tbh.
If a game has a city where you see other players, trading, group play, reputation, and a world to explore it is called an MMO. It can be the exact same game except we see people in Tristram, we can trade, and we have reputation. The world in D3 is already open to explore as one unit, so nothing would really "need" to change in that regard either.
As Munchen has said, this cycle of "good kills evil, becomes corrupted and the new evil, rinse & repeat" has been driving the entire franchise. I'll just post the D1 ending cinematic because I guess 20 years later, many people have either forgotten about it or never seen it (WHAT A SHAME! ;-)):
To me personally it would be a bit odd and almost boring if it would just happen again a fourth time. It wouldn't be all that unusual in this day and age, where reboots of franchises are done quite often and aren't even telling a different story (Spiderman, Star Wars, and a million others come to mind). What I could imagine is a PvP-oriented spin-off. I wouldn't like it, since I have no desire for PvP at all, but I know it would be damn successful because I know that I'm an outlier and PvP is what drives many people. So yeah... I could see something like what you mention, an MMO or Overwatch-style game where you choose sides, either play a barbarian or Diablo. (Not sure who'd choose the Nephalem side though, wouldn't we all like to be evil just once :P)
I feel like the next easiest/logical step is to take D4 into the MMOARPG realm. As Demonmonger pointed out earlier in the thread, if you shift focus from endless grinding for min-maxing and move toward grinding for rep, crafting, rep and crating rewards, boss/dungeon farming, etc with more of a social environment rather than single instanced games with four people - you'll get a game experience that is following some of the ideas of international games that are getting a lot of traction (Lost Ark, Wolcen, etc)
This type of model is definitely in their wheel house at Blizzard and they can throw that experience behind it.
Hint, hint: the lead developer for WoW just moved from WoW to something else within Blizzard...
Shit. Had totally forgotten about that. Pretty clearly leaving a huge opening for some sort of continuation. Maybe a corruption mechanic in expansion? Who knows, but worth pointing out. They obviously had something in mind when they wrote up this hackneyed ending. lol