62 BC the Gauls invite Ariovistus in to help one side in their civil war, which later makes Caesar pop by for a visit and the Res Publica Romana fall...

The Suebi would be strong at this point, but here or pretty soon after, though perhaps more under the pressure from Rome, the Cherusci and their allies become a power. Likely this was more a question of unification as a result of war. An early example of the simplified "States make war- war makes states" phenomenon. The Boii and Dacians/Getai would still be around, though the Boii in gradual decline and eventually gets taken over by the Markomanni.

Or you could choose slightly before with Rome in civil war and Mithridates still alive and kicking.