Okay, so it sounds like we're drifting away from the initial court-intrigue place where we started? Moving toward conqueror? Maybe we can split the difference...
A Conqueror coming for everyone is a very straightforward type of conflict, which isn't necessarily a good thing when you're trying to make grist for the mill.
If all of the PCs are Nobility of the same Kingdom, there aren't perhaps very many circumstances for you all to come into conflict with each other. You all would have one definite and obvious foe that everyone would have pretty straightforward need to work together to keep the Conqueror out. That's a little too obvious. It's a more traditional rpg big picture. We want complexity, and many possible angles, to the same conflict. In Burning Wheel, while we want the PCs to ultimately work together, we want them to have subtle conflicts within that as well. We want that grist for the mill.
Let's look at maybe combining the Conqueror-angle with the Civil War/intrigue-angle?
Maybe for a long time - several generations, the "Horse Lords" (or whatever) have been divided, each sovereign over their own Domain. But recently a figurehead has emerged who claims that they want to re-unify the Lords, they want to become King of the "Horse Folk" like there was in the days of old, in the Glory Days of the Horse Folk, when Domain of the People as a whole was much further reaching, held over many foreign lands? Which would also mean a single sovereign... Most of the Noble Houses might understandably want to cling to their sovereignty, but many people - even many in high positions in the Houses maybe - could also yearn for the Glory Days, and would welcome a return to the Old Ways, to the One King? Whether for the greater glory of the People, or whatever. So there is a conqueror but there is also room for intrigue.
Quote:
...would it be wise to have another dimension to this? Maybe a third party or a wild card? Maybe an outside threat...
I like the idea of the wild card, but I think for the Big Picture, it is a looming thing in the background. And I think the plague idea mentioned already would fill this role. Maybe it's a true natural plague? Maybe its magical? Maybe some people believe its a curse, brought on by the disfavor of your Ancestors, for forgetting the Old Ways? Maybe it's only the first manifestation of something greater going on? We don't know, and that we don't state that in the Big Picture, it's the part that's out of the PCs control. For now.
So how about the Big Picture is something like (and we can come up with good names and terms later if we want):
"Civilization is waning, a plague has stricken the Fleets of the Horse Lords, and malice is growing in the shadows. A woman has emerged, claiming to be the rightful heir to the Unbridled Throne. The Queen of the Horse Folk, as she is called, is now riding on each of the Domains of the Horse Lords with her Thousand-Fold army, one-by-one, to either receive their fealty or take the heads of the Lord. And now she rides on Our Home."
Is that interesting to people?