If you guys want to see a real Diablo MMO, check out Lost Ark: https://www.youtube.com/watch?v=LL5zGvsmSM0
Unfortunately, it's Korean-only, but it could be coming to western shores, if successful.
Like danutz, I don't think Blizzard will be doing a full blown MMO again. It's much more expensive, and risky, than the other genres they're now dominating. If anything, I'm guessing they'll do some more MOBA stuff, as HotS (unlike Overwatch, SC2 and WoW) isn't topping its genre.
What made you feel Diablo 2 has a cartoonish style Bobo?
If they do the same lazy thing with it as they did with StarCraft - then it's nearly pointless.
Diablo 2 definitely had a cartoony style - but it was a strange sort of hybrid, and the game was full of gore. But I prefer the original Diablo aesthetic overall.
Well, not entirely - because it's a pain to look at today - but without re-rendering the original assets, it won't be that much better. The color palette and resolution of the assets are both painfully inferior to modern standards.