As someone who only played Diablo 3 a little bit, I'm still not sure what a Diablo 4 would be that wouldn't just be an expansion for Diablo 3. I expect it to be pretty much the same graphically, the same gameplay, etc. Diablo as a franchise just seems creatively out of ideas unless they do a complete 180 like making it a strictly solo-player experience ala Dark Souls or first person or something.
There doesn't seem to be anything there that is worthy of a new entry in the series.