Hellgate: London was a game that experimented with this style of view (though it was first person was available too). It was an ARPG made by some former employees of Blizzard North that left to make their own gaming company.
While the game ultimately failed, the first person view was really immersive. In reference to Lamack's list above, there were still large packs of mobs to aoe down. Speed builds were still popular for farming. Teleport skills still existed. The aoe effects were quite impressive (for the day). Pet builds were very viable (even a little TOO viable in some settings). There were more NPC's than in diablo, but that was for story reasons, not for gameplay reasons. Environmental factors were much MORE influential than they are in diablo (cover and elevation were very impactful on what builds worked at high difficulties).
There's no reason an ARPG cannot be made with that camera perspective and still maintain the mechanical variety we expect from an ARPG.
It will, however, have a dramatic impact on the perception of your character. A first person or over the shoulder view makes us view the characters as embodying ourselves. It's a much more personal style. Isometric views tend to have us think of someone controlling a character (as opposed to being the character).
I would argue this is a core aspect of the Diablo series thus far. Changing the camera view would shift the focus away from the world and towards the character. This shift in focus, both literally and figuratively, would be a gamble. But it isn't necessarily a bad one. Different isn't always worse. Blizzard certainly has the resources and talent to pull it off well if they desire to do so.
In the long run, I don't think simply blending the good aspects of diablo 2 and 3 will work. Games have progressed far too much in 20 years for that. If/when we ever see a Diablo 4, it will have to be a fairly substantial evolution of the series. A new camera perspective is likely first of many potential changes that will have to be weighed.
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I dunno why you guys are getting so worked up about a book cover. Clearly this is the cover to the next Diablo novel or a new candy crush variant called: soul stone crush