The river Seine parted the Belgae from the Galli. The river Visurgis parted the Istaevones from the Hermiones.
I'm pondering a possible diversion creating an Imperium or Confoederatio around the Istaevones, Belgae, Batavi and Ingaevones west of the Visurgis, using the rivers as outer borders based on the above. Weser, Lahn, Moselle and Seine as the principal outer borders. I.e. spanning from Lutetia in the south to Phabiranum in the Northwest, not including the Agri Decumatum or Alemanni. Centered around CCAA, obviously.
In the first or second century, if that is unclear from the above.
The question is when exactly? Somehow, the only possible divergence points i could use in my opinion is the Batavi Insurrection in 69 AD (Quintus Perilius Cerealis failing?), as i'd want to be past 50 AD for CCAA to be already colonia. Another possible point would be 16 AD, Germanicus' invasion of exactly that area, but i couldn't see any possible storyline leading the area to independence of Rome, considering Germanicus was of the gens Julia.
Anyone have any other ideas, possible in the 2nd century?