Well, since you asked:
Before investing time, resources, and cash to a project that is a such a departure from the franchise, I would take the pulse of the community and try to gauge what the Diablo community is looking forward to. Granted, forums can only tell you so much, and don't represent everyone, but it is a start.
Next, I'd try to find out, in as quiet a way as possible, what type of reception the game might receive; note, a new PC/console game, despite the setting in the game's timeline, would not necessarily need this type of scrutiny; if I was on the Blizzard team it would never enter my mind that Diablo fans would not want a new PC/console game. How to achieve this? You can do surveys, polls, deep research. This involves cost as well, but I would imagine a big investment is worth the, well, investment.
So, and if the reactions since yesterday are any barometer, they would have seen that the game, at best, would not be what the part of the Diablo community that engages online is looking for, and at worst would be hated. Now, say they still wanted to go with it AND they have plans to build on the PC aspect of the game with a new entry in the series; how would I introduce it? This is what I think:
I would have revealed the game at a different venue than Blizzcon, maybe Gamescom for example. I would have promoted it more as a companion to the series, and started teasing that "more is coming". I would have marketed it as a way to enlarge the brand to new fans, knowing that the core fans would not have a great reaction to it.
I would not have made Diablo the centerpiece of the opening ceremonies, placing it on the Mythic stage and allocate the first panel of the day to it. Granted, given the other reveals for the other franchises, there isn't much else that was really mindblowing, so maybe that is a little unfair.
So that is my take. I do not work for Blizzard, don't know everything about the business, etc., and I know that armchair criticism carries its own irony, and, I do not want to slam Blizzard for their creative efforts; I just think they chose the wrong direction for the franchise.