I'm thinking that a large reason for the success of 5E is that it appeals to both old and new gamers alike. It harkens back to old school flavor and rule systems, maintains the design consistency of 3E/4E, but is also simple enough for new players to start. In that sense, focusing on Greyhawk would be a mistake, perhaps a colossal one. Let's be honest: Greyhawk is mainly beloved by older gamers, those who came of age during the halycon 70s and the boom era 80s. It has a lot of nostalgic cachet, but probably not a lot of appeal to younger generations and new and future gamers. Certain aspects of its flavor (e.g. its names, as someone mentioned) are just too idiosyncratic.
I do have a few suggestions of possible approaches that could be taken. It seems the basic problem is two-fold: whether it is worth the resources to allocate to a setting, and which setting to choose. I think the answer to the first is probably not; the second is unanswerable. This is probably why we haven't seen a setting yet, aside from the SCAG. But there are other options.
Several years ago--I think in the early days of 4E--I said that I would love to see a hardcover book called Worlds of D&D, or something like that, which described every official D&D setting, major and minor, in broad terms. If it was 320 pages, you could have longer sections--32 pages--for a few choice settings (FR, Greyhawk, Dark Sun, Eberron); shorter sections--16 pages--for the secondary settings (Dragonlance, Spelljammer, Mystara, etc)--and then very short sections--8 pages--for the minor ones (e.g. Al-Qadim, Jakandor, Ghostwalk, etc).
Of course there are problems with that approach: For one, you just won't be able to go into enough detail to make the settings all that useful, and it doesn't help fans of specific settings who don't want to buy a $50 book just for 16 pages. That leads me to the second approach:
Write a series of Classic Setting Gazetteers, probably not needing to be more than 64 pages long, which provide a guide to how to play the setting, a few specific rules, broad info on the world and detail on a starting location (e.g. Shadowdale). Finally, each gazetteer would have a few page appendix that described the publication history and info about what older products would be particularly useful for more information.
To me this would be the ideal approach. I'd love to see an aggressive publishing schedule: maybe once every quarter, so four a year. If it is too much work for the skeleton crew at WotC, they could outsource to different publishers like they did with Tyranny of Dragons. I mean, why not? If any of the gazetteers sell really well, they could start a whole line of products. They wouldn't need to do FR, as they already did SCAG. A possible order of publishing might be: Greyhawk, Planescape (Sigil), Nerath (Nentir Vale), Dark Sun, Eberron, Dragonlance, Mystara, Birthright, Spelljammer...or something like that. Not sure Al-Qadim, Ghostwalk, Jakandor, Council of Wyrms, etc, would be necessary.
Anyhow, just dreaming...
