This is something I once looked into...
... If Varus had held Arminius to battle and defeated him, Germania could have been occupied as far as a far more defensible frontier from the Elbe through the Harz Mountains to the Erzgebirge and the Danube.
That may have been Augustus's plan and (read Strabo) might have been achievable. The knock-on effects would be that the Imperial Limes would have needed fewer Legions and Britannia (and Hibernia) might have been completely held. I imagine the Legio XX Valeria Victrix staying in Inchtuthil in Caledonia, the Legio IX Hispana in Eboracum, the Legio XIV near Dublin and Legio II in Isca. No Irish Question - only the Pax Romanum.
Thoughts?