Nothing, which is why people keep suggesting big picture alternatives instead of working on what we really need to start working on--the fine details of one, definitive narrative.
Which obviously I think should pretty clearly be the Moonrise plotline. Nothing else that's been suggested is as interesting as that idea, and most of the alternatives would result in the main thing that makes Starstill an interesting plane being destroyed, which is something Wizards has already pretty much determined to be a lousy way to resolve block plotlines.
I mean, we haven't even definitively settled on the dragons being Lesser Elders! That's like... huge key stuff that should really be set in stone so we can move on.
Well then stop shooting down my ideas
I think the plane was stopped for a good reason (i think it would be interesting to say that it was to avoid the eldrazi), and that it is currently unable to be started again because people are unable to make that kind of sacrifice again.
If the dragons gave up something in order to stop the plane, they don't have it anymore. The best thing I can think of is a planeswalke's spark. Some planeswalkers might not know this, other planeswalkers might recognize that this is the sacrifice needed to start the plane again, but are unwilling to do so. That could make the dragons angry, that other planeswalkers could be so selfish compared to what they, the dragons, gave up.
I'm not saying that there isn't another way, but that also doesn't mean that the Moonrise will definitely work. I think it should be an idea that doesn't work, and then once people discover that it doesn't work everyone starts blaming everyone else, and it becomes a planar war of who's fault it was.