I've often concepted dynamic conditions that would cause such a migration. Say, if the homelands of the Cimbri or Teutones were occupied by a non-Sweboz faction, or even perhaps the Sweboz if the right kind of government were installed, a mass exodus would spawn, with messages indicating such to every player in the region: "Imperator/Chieftan etc, an entire nation of people have begun to travel because of ..." and many (5 or 6) full stack armies would spawn.

Problem is, we cant make spawned armies do anything, and usually they just sit around. Maybe in M2TW.