Fortress Legends (now defunct) actually did d3 style gameplay pretty well, and it was years ago. I reckon they *could* do diablo-style gameplay on mobile, it just depends if they do it right.
http://www.bay12forums.com/smf/index.php?topic=157009.msg6879806#msg6879806Admittedly, this was all "user-generated" content, but that's actually harder to do than procedural (there's an entire back-end needed for people to make their little fortress deathtraps for others to raid). It only had 4 skills (choosable, and very different skills depending on class) and basic attack, but it was pretty fast and fluid at what it did. Controls were responsive, even during lag, and tactics and builds were various enough that you thought "oh, good, we've got one of them on our team this raid. Sweet. I'll mix my own tactics up a little."
And it was free and fair. Always online, almost always multi-player, but it was even pretty good at that.
So it's doable. It just depends on whether or not Blizzard has any intention of making it good and long-lasting and not just a cash grab.
Fortress Legends is still some of the most fun I've ever had in a mobile game, ever, bar none. If anything, it was too free and fair, so Bandai Namco had to rip the servers eventually. You didn't have to pay squat to become a pretty powerful character. But wow. If Diablo Immortals has even a drop of what that game did, even without all the BS timers and "create your own dungeon" thing, I think they can do it.
It was a flash in the pan, but that game reminded me more of d2 than any other action RPG on mobile that I've played before or since. And I've played a tonne of them these days, and gotten rather good at some of them. But if Blizzard can do something remotely similar to Fortress Legends, I'm in