They needed time, and did not needed an expansionist imperial state from the Mediterranean with the capability of destroying them all or compelling them to ally with it.
The Arverni, between the late Third to early Second Century BCE dominated the other Gaulish tribes in a large hegemony stretching from the Atlantic to the Rhine. Similarly to Rome's relationship with the other Italian states between the Fourth-First centuries BCE. If it was left unmolested long enough, the Arverni could have gradually unified Gaul under its leadership. Obviously, it wouldn't control all the Celtic tribes everywhere, but it may have had the strength to eventually project its power beyond its own immediate surroundings.
I'd agree with this statement. The Arverni are a great bet for a strong, centralized Celtic state. The need only to keep the Germanic tribes at bay, and avoid any large scale wars with a unified Italian state. If they get away with seizing Massalia, they have a major Greek port along the Mediterranean coast, which will give them direct access to trade with other civilizations, increasing their wealth and legitimacy.
In Iberia the Celtiberian Federation, the Lusitanians, and the Asturians also have a chance to become powerful Celtic states. They, too, would need to keep Rome off their doorstep, but also would have to contend with the Carthaginians, specifically the Barcas. It's hard telling exactly what will happen there.
What happens in Britain is hard to say, and honestly depends heavily on what happens in Gaul.
In the Balkans, as usual, it would be chaos. There were Celtic powers like the Scordisci and the Boii in the area, but they were caught between a rock and a hard place with the concurrent rise of Rome and Dacia. Perhaps with some tweaking, they could make it out as very strong powers in the area, though likely unstable.
Then there are the Galatians, which, to be honest, don't look like they could last much longer than they did. They were too isolated, remote, and frankly they pissed too many people off. It almost seems inevitable that by the 1st century AD someone would gobble them up.
Another interesting prospect would be the idea of messing around with Celtic mercenaries. They were very popular in the ancient world, used by armies in Egypt, Carthage, Macedonia, Syria, Italy, Iberia, and Greece. If you have one particularly treacherous and ambitious band of Celtic mercenaries, they could overthrow a power and carve out a Celtic dynasty in an exotic land. If they are xenophobic enough, they could preserve their culture amongst their own aristocracy in the same fashion that the Ptolemies did.