That does sound interesting!
From what I understand based on some leaks, an early project for a potential sequel in the Diablo franchise did involve a third-party camera system and souls-like combat. It was code-named project Hades. Reports didn’t say whether it would have had MMORPG elements, as their last published attempt of a sci-fi MMO was canceled for reasons unknown, but that would have had more like the over-the-shoulder action-oriented combat that you’re talking about. Kotaku’s article titled ‘The Past, Present, And Future Of Diablo’ talked about, but I’m apparently not allowed to link it here so you’ll have to Google it.
It did sound cool. I had some dreams about playing a Diablo-esque game in that style, but alas, at the time it was not to be. There’s still the possibility that they may explore different directions to take the franchise going forward. We know Blizzard has also experimented with bringing other franchises to the action genre as well, like StarCraft Ghost.
At the time, Souls-like games were a pretty big deal, and would have been very fitting for the Diablo franchise. Nowadays, several gothic/medieval-style combat games are starting to explore some gritty, first-person games in that style as well. While a Souls-like would likely have been slower paced, games like Witchfire and Unawake seem more fast-paced. I could totally see a Diablo game in that style being pretty awesome. Or even a horror style like Dark Moonlight where you try to escape from monsters. Either one could be really awesome if done right, and I totally agree that the Diablo universe could make for a great MMO as well!