Unless they have ideas for system changes that would necessitate a whole new game, I'd rather they just continue adding on to D2. And honestly, regardless of whether or not they release a D3, I think D2 is still going to continue to be supported with new content for a while, especially now that they don't have Activision funds. Plus, now that they don't have to have a big new thing ready every September, they might be able to take more time to make what comes next exactly how they want it.
In an ideal world (and I don't think it's too far-fetched an idea), we'd get a second annual pass that begins in September, and on top of new content like what the current pass has given on a regular basis, it would also have new story missions that would be added every week or so. Make it almost like an episodic model— check in every week to see what direction the story is taking, and then once you finish it, you spend the rest of the week playing around with the new gameplay additions, on top of the usual weekly milestones and everything. When the pass starts, a new area (like the Forgotten Shore) is added, and maybe a second one gets added halfway through the year.
