See... while I agree for the most part with "A really good adventure"... in truth, I think we'd be better served with "Several really good adventures from specific campaign settings that includes player options to update characters to 5E rules."
If the era of the splatbook is gone as Mike has hinted at (where most of all the first new books released are all just player crunch)... and the idea of releasing new full-sized Campaign Setting books that end up re-writing or kind of adapting previous material is unrealistic because of the size and amount of details necessary to make them actually useful...
...I'd rather see them release a series of Levels 1-10 adventure modules plus mini-campaign settings (a la Ghosts of Dragonspear Castle) for each of their main settings that include the player crunch necessary to adapt characters into that particular setting. So the Forgotten Realms adventure would include race information for genasi and drow and the swordmage sub-class... the Eberron softcover would include the shifters, kalashtar, and changelings plus the Artificer sub-class... Dragonlance has kender... Dark Sun has muls and thri-kreens... etc. etc. Give us a mini-campaign of adventures to take us from Levels 1-10 with all the details of the small area we are in, and fill in the character creation details to cover those options not already detailed in the main game.
At this point I don't know if any of us really need full campaign setting books covering the entire world of these settings (since we've gotten them several times before), but perhaps just a smaller area in much more detail would serve. And we also don't need books of just crunch, because that tends to expand the rules of the game too far too fast. And while good adventures are absolutely important, those by themselves tend to often be DM-only purchases and thus aren't as financially sound as other options.
But if you could combine all three of them together into a single product? A product that both players and DMs would potentially buy? A product that opens up most of (if not all) of the game's previous settings within the first year of the game's existence? You might have something that gets a huge swathe of the gaming population up and running in their favorite settings using 5E right off the bat. And even if the mini-settings do nothing for some people... you can still easily yank out the adventures and place them in your own individual world like always.
This seems to me to be the best of all worlds.