A couple ideas that come to mind:
One, I feel like a truly great dragon-slaying quest has never really been done, or done well. I'm not sure how that would be done over 20 levels, but I'm sure that it is possible.
Oh, I like that. Though I think a Dragon Hunt on its own is probably not something for 20 levels. But why not kill a few more than one. Start as a Dragon Hunting expedition that reveals some sinister plot involving multiple dragons, and some other factions - so that not every adventure is a dragon at the end.
As a perhaps side tangent, I think one of the ways that 4E failed is that it didn't provide a cohesive and fleshed out campaign setting for folks to call home and/or draw inspiration from.
I love the Point of LIghts setting and that it isn't cohesive and fleshed out, but a collection of plot ideas and historical elements you are free to interpret on your own. I like to believe that the people that used POL were able to take all these elements and spin a setting that genuinely feels their own.
Eberron, Dark Sun, Forgotten Realms and Greyhawk all feel more like someone else's setting to me, and I worry i may "break" the setting accidentally. PoL made it clear that I am free to interpret the different events and individuals as I want to.
With that in mind, imagine an early box set product that was akin to a sandbox campaign for low level characters in a Nentir Vale/Dalelands-type setting, from which numerous adventure paths/branches could sprout. This, I think, would embody the modularity idea that is intrinsic to Next.
That, on the other hand, I like. It could have been done in 4E already, but they choose to do only a single "path". Having different turning points where you can go either adventure A or adventure B sounds interesting, and could even allow a certain replay value, or sharing the DM responsibilities - imagine splitting groups every so often to explore 2 alternative adventures.
As long as it doesn't have kobolds, I'll be good to go. No kobolds, please. I'm sick to death of kobolds. There's always kobolds. What's with all the kobolds?
Sorry, at the current pace, it will take at least 3 more D&D editions to eradicate all Kobolds.
At least the low level ones. We have to expect that evolution will create some particularly vicious Kobolds that will become high level enemies. They will probably use Ultra-Dire Warspider Lords and Super-Vicious Plague Rats as mounts.