Back in the late 60's BCE, there was a long-standing rivalry within Celtic Gaul between the Aedui, who were building an empire of sorts, and the Sequani and Arverni, who were, in turn, threatened by the expansion of the (Roman-allied) Aedui. The latter party invited a king of the Suebi Germanic tribe, Ariovistus, to cross the Rhine and join them in an encircling movement to defeat the Aedui once and for all. They succeeded, and one Aedui druid was sent to Rome to appeal to the senate for aid. A general, one Julius Caesar, decided to go and help defeat Ariovistus' expanding hegemony over Gaul. Caesar eventually defeated Ariovistus at the Battle of The Vosges (58 BCE), vanquishing the Suebi. However, the Gallic tribes began to feel discomfort over Caesar's continued garrisoning of the region... and the rest is history.
But what if Ariovistus had managed to defeat the Romans? Say, by another, less competent general being sent instead of Caesar.
Could his Celto-Germanic kingdom continue growing in size?