As the old adage goes, if it ain't broke...
Diablo I/II worked - extremely well for what they were. I can't imagine Blizzard deviating from that in any way, shape, or form. Updated graphics, support for widescreens, and a new story would be all that would be different. As for whether they'd attempt a pay-to-play model now, who knows. I probably wouldn't subscribe, because I don't want to pay to play co-op with a few other random people who are most likely jerks.
If they keep the formula the same - Battle Net included - they'll have another smash hit on their hands. And I'm really not worried that it's the original team anymore. Blizzard has shown they have the chops to do it. Just look at the POS Roper and co. produced. I have faith in Blizzard.
"He was one of a kind. He was a mutant. A real heavyweight water buffalo type...who could chew his way through a concrete wall and spit out the other side, covered with lime and chalk, and look good in doing it." - Bill Murray as Dr. Hunter S. Thompson -
"I like dogs playing poker... because dogs would never bet on things; so it's incongruous. I like incongruity." - Christian Wolff