With the rumor mills buzzing about a new Diablo game in development, and given that I've been playing a lot of Diablo III lately, I thought I'd reflect on the possibilities for a new game. Season 5 has invigorated me to the point where the stuff my main-game characters will be inheriting will be far, far, like exponential to several degrees better than what they had before. (I've got full sets of gear for most of my non-seasonal male/female counterparts to suit them up in.)
Now, the big, huge caveat here - this could be another expansion. And I'd be ok with that, as I'm rather fond of my characters (though the introduction of Xul in Heroes of the Storm makes me really want a Necromancer - even if the Witch Doctor mostly covers that niche,) and I'm always happy to have Lyndon along making quips.
Setting the story aside (since I kind of covered that,) one question might be how the mechanics would change in a future game.
Diablo III gave each class its own resource, and while a lot of them work pretty similarly, it's a nice way to distinguish their mechanics.
The Adventure Mode introduced in Reaper of Souls gave players a ton more freedom - you no longer had to endlessly cycle through the main story and the same dungeons and bosses in the same order over and over.
I might try to work this freedom of exploration into the main game. It might be interesting to have a number of threats (like Belial and Azmodan in Diablo III) that you can take down in the order you want.
Personally, I wouldn't mind seeing the loot become a bit less random. Basically any item can drop from any enemy (though some require you to get them in rifts) and while that gives you more of an incentive to go clear out that Khazra cave, it also dilutes a bit of what's exciting about fighting a boss.
The way Legendaries work in Diablo has you really get tremendously more powerful once a particular effect is gained - as an example, if you are using the Thorns of the Invoker set on a Crusader, Belt of the Trove suddenly has you raining down Bombardments constantly and thus not only getting a lot of free damage (and freeing up an ability slot) but also perpetually having your 50% damage reduction buff.
But once you have that set, with all the best legendary effects, your main incentive to keep playing is simply to amass better gear for your off-sets or alts. Or grinding legendary gems to help you run higher Greater Rifts.
It might be nice to segment the end-game at the level cap a little more, and to provide quantitative differences after getting qualitative ones. Maybe you can't get gear that would let you face, oh, I don't know, some Nephalem who has gone over to the side of Hell (brainstorming potential villains) until you've gotten a decent set of gear together.
Similarly, I wonder if it might help (or hinder) the game if getting your particular legendary effects was less difficult (or tedious,) but then there might be several levels of "Ancient" gear to work your way through, allowing your set to work properly but gaining new ways to increase your power.
Diablo seems to be all about slaughtering massive swarms of enemies, and I think the tried-and-true isometric camera is probably right for that. If they want to scale things down a bit and make each enemy count more, you could do a WoW-style ground-level camera. But that's probably not the direction they mean to take for the game.
Obviously, a brand new game would need brand-new classes, though even an expansion would probably require at least one new one as well. WoW's classes were partially inspired by Diablo II's (as well as some WCIII hero units,) but you could almost kind of reverse the process.
Obviously there are some clear 1-1 correspondences: Paladins are Crusader, Warriors are Barbarians. Monks are Monks. Mages are Wizards. Ironically, Demon Hunters are not Demon Hunters. Diablo III doesn't have a real Druid equivalent, unless you seriously tried to stretch Witch Doctors because of things like Plague of Toads and Corpse Spiders. There's also not a really clear Cleric-type, though Crusaders' spells do veer into that territory. Demon Hunters kind of cover the stealthy-rogue combat style as well as the ranger/hunter archetype.
Of course, a Diablo IV would be able to toss out all the Diablo III classes and begin anew, freeing up a lot of the archetypes that the current classes fill.
I suspect that we'll get our first real news about this new game or new expansion at Blizzcon, though given how they did Legion and Reaper of Souls, it may come sooner than that.