Give me D4. I want an entirely new game, and something other than Rifts, rifts, and more rifts in a game.
That is an interesting point to discuss. It's easy to agree with that (as was my first intuition - running rifts and rifts and rifts is boring), but then, what's the alternative? Running the same areas over and over again as in D2 was even more boring. I personally think that rifts are better than re-running the story countless times.
So I guess there are three options:
1) Change rifts to be more exciting. Have the player add certain effects to a rift instead of making it purely random, so they have a more unique feel. There are so many ways this could go. One important consideration here is that it would mess with the competitive aspect (i.e., fastest rift clear of highest difficulty as being the best) - getting rid of the leaderboards would be great. Instead, what if the leaderboards would instead measure "drops per hour" or "monsters killed per hour"? Or no leaderboards at all?
2) If we abandon rifts, what else is there to do? Falling back to running Crater or bounties or farming bosses is certainly not what anyone wants (I hope). So if it's not repeating a certain thing (whether it's called rift or whatever), we'd have to change the game entirely. Do we still keep the item grind at the core? Look at games like Clicker Heroes which are a million times more "dumb" than rifts, but still being played by millions of people. I think there's no issue with grinding, as long as the rewards scale in a good way. I think the hunt for rare items and runes as in D2 worked better in that way than the paragon/augment farming. A certain amount of lottery involved needs to be there. Blizzard argues "people need to have the assurance that after playing for an hour they'll get a reward". I call bullshit on that. Constant reward is work (i.e, your daily fixed paycheck for putting in a constant effort). Fun is feeling lucky, winning the lottery, winning against all odds, receiving stuff you didn't expect. There's nothing unexpected about paragon or augments. No one goes like "HOLY SHIT I GOT ANOTHER PARAGON LEVEL, THAT'S SO EXCITING!". So keep the grind, but add different rewards.
3) Revolutionize the game. This comes at a cost: we'll leave the ARPG genre. Some might like it, but we'll without a doubt lose some people. Just like Fallout became one of the top games with its re-vitalization by Bethesda, the game is now super popular and has certainly made great strides forward. I personally got lost - FO3/4 aren't for me. That's fine. I have to accept that a Fallout 2.5 is something that is too niche. Just like Diablo 2 Remastered would only be played by a handful of people (if you think it would be a game for the masses think again; StarCraft Remastered has shown us how you're in for a rude awakening once the nostalgia is gone, which was the case for 90% of people). Diablo 4 could be a mobile-based game like Pokemon Go. It could be an RPG and get rid of the action part. It could become a card game full of microtransactions (oh wait, there's already Hearthstone). Or it could be something entirely new no one is thinking of!
Even if Diablo 4 is already in development, and even if it is being announced next Blizzcon, it'll take years until its release. I think anyone who believes there is even an opportunity of playing Diablo 4 before 2020 (and I mean release, not beta) is delusional. I personally believe that something akin to Diablo 4 is in development - but it might not be Diablo 4 because I remember them somewhere saying that Diablo is sort of a trilogy. They also need to fundamentally change the franchise to get rid of the old stench that causes all the nostalgia-infused, unjust criticism to any new installment of the series - and the best way it so re-invent Diablo from ground up. What that is gonna look like no one knows. I personally hope Blizzard goes back to what they have done in the 90s: inventing or reinventing game genres.
At the same time, they also hinted that internally they had other classes to choose for the character pack, but decided to focus on the necro (first?). Of course there is the possibility that the necro was just the additional class for the second x-pac and no other class is to follow; but everything they've set up - the implementation and style of releasing the necro - makes it possible to "copy" the process for other classes. If my memory is fooling me some of the designers already talked about other classes they liked. So no matter of what's in development, I believe that other classes are to come; designing an additional class and selling it as DLC is a pretty lucrative thing. So lucrative that it's the main income for every other Blizzard game (see at which speed they're churning out co-op commanders for 4.99 in StarCraft 2).
So imho: both.
Also: please don't capslock, I've edited your title