The cities in the Po valley are far enough away from both Rome and the Etrsucan heartland that they aren't in immediate danger of being swallowed up. As to how they become powerful enough to rival Rome, I think that this is where the Gauls might play an important part: Rome was sacked by the Gauls led by Brennus in 390 BC (Supposedly, some say 381 BC or even later). Eventually, these Gauls displaced the Etruscans in the Po River valley as the dominant group, thus Cisalpine Gaul; my idea then is to have the Po Etruscans survive and integrate these Gauls.
The effects of this would be two-fold IMO: first, the cultural changes that such an integration entails helps me rationalize why the Romans won't simply gobble up these groups the same way they did the Etruscan heartland. Second, this would have huge ramifications for Gaul later on, if they remain in contact with the Cisalpine Gauls: the Etruscans had a society that was, for the most part, on par with the Roman Republic, at least in terms of government and culture, which the Gauls might be willing to adopt, and the system of government of the Etruscans (independent cities united in a loose confederation) is something that the Gauls would be familiar with.
The Gauls that sacked Rome between 390-87 BCE were the Senones, whom were located in Umbria. The Celts that conquered the Etruscan territories around the city of Felsina (Bologna) were the Boii tribe. Milan was founded by the Insubres, related to the Aedui of northern Gaul. Therewas the Cenomani, possibly related to the Aulerci (Le Mans) in northern Gaul, the Lingones in Ferrara, the Sallasi in the Aosta Valley, and the Celto-Ligurian Taurini in Piedmont. But the Senones under Brennus, the Boii in the Po Valley, and the Insubres of Lombardy could potentially have made a more lasting impression in ancient Italian history.
Brennus could have alternately completed his siege of the Capitoline Hill, destroyed the Republic, and relocated his people from Senigalla to Clusium and created a new Celtic state encompassing Etruria, Umbria and Latium.
The Italian Boii had already captured former Etruscan holdings, and would have adopted early on some of the more practical aspects of their urban culture, as the Ostrogoths had after their conquest of Italy centuries later.
The Insubre of Milan were part of a greater confederation of Gaulish tribes headed by the powerful Aedui in Saone-et-Loire.