As others have speculated, if Sojourn survives long enough to grow and unify a swathe of Xaos, it could be a post-Hegemonic ally on the southern border that supplies you with alchemical components you’d otherwise need to bring from Erezza. Or it could just be a drain on the Hegemony’s own military resources. Or, yes, an enemy with numbers swelled by hungry Southriding refugees that bites you in the back. 
Another reader queried by PM:
You aren’t the only rebel, or the only reason the current Hegemony is tottering. There’s no way to keep the current regime from collapsing at the end of G4. It will be possible to revive the Thaumatarchy, but that will inevitably involve some rebuilding and adaptation, not just taking over the current system.
Yep. Refugees eating farmers’ seed stock and unripe crops in the field, raiding every barn, stopping every wagon or barge to search it for edibles, sending farmers fleeing to the cities to petition for redress…your border zones aren’t going to get much of a yield for a while.
That partly depends on the Theurgic balance of power – remembering that cavalry is much more vulnerable in the gameworld than in ours – but yes, especially if the urban Nyr faction loses out to the nomads, and the nomads then draw a substantial share of Nyryal’s 3,700 Theurges, we could see Nyryal turn into magic Mongolia. And if most of Karagon’s Theurges end up joining an exodus or concentrating in the cities closest to Shayard, the Nyr nomads could go through northern Karagon like a knife through butter.
Not until some G4 conversations with Sarcifer, I don’t think. It’s possible that the well-informed Nyrish sage Harza you meet in G3 will have enough of a grasp on the dynamics to give you the first hints, but really, I think Sarcifer is the first person you’ll talk to who has the info to really open the MC’s eyes to this.
Got to run now – more responses later…