I think the key thing that is weak is assuming that the Core Story needs to be about just one world.
The point I'm making is that once you play a campaign of Burning Empires - what next?
The benefit of mainstream games is that they are better suited to many different types of games that go beyond a singles structure.
BE, on the otherhand, risks becoming more of the same. So for a year, it's great. But what is the likelihood that the next game will be different enough. It just becomes another Vaylen invasion.
A supplement that allows you to then go beyond that would be a great idea. Because otherwise interest in BE will dry up. Eventually everyone will have played a game and started looking for something else.
I'm kind of playing devil's advocate here - I think that the core system has a lot of flexibility - but it does limit things to one world per campaign and has no way of carrying events over from one world to the next.
Just a thought.
Conan