Well, it's really hard to answer there.
See, Celtic civilisation was really proteiform and some peripherical regions were culturally distinct such as in Britain, Iberia or Galatia. Giving the different super-strates, sub-strates and various influences that could play there, a single answer for all Celts is more or less impossible to seriously pull.
Assuming we're talking only of Gallic peoples, it's not really simpler. Gallic tribal states and chiefdoms were more or less deeply influenced by Rome (and earlier, by hellenic influence coming from Massalia and the harbours of modern Languedoc) up to their institutions (such as the decline of druidism and the rise for vergobrets in the IInd century BC).
Depending if Rome never shows up at all in Gaul or even as a regional power, or if it does show up as a big influence but for some reason never really conquers Gaul.
If the former, I'd guess that the hellenic influence and the absence of a strong regional power (let's assume, for exemple, that Carthage wins the IInd Punic War, and Rome remains stuck at least for the time being as an italian power) could help preserve the druidic institutions, altough not without a significant change IMO.
Giving the huge roman influence on late IInd and Ist century Gallic society (such as economical "unity" and the appearance of Roman-inspired institutions such as vergobrets), butterflying it could
Arverni are likely to be unchallenged in the IInd century, with their hegemony spawning from Mediteranean coast to Limousin, especially if we accept the idea that Elysices were crushed by Carthagians or rather, allies of Carthagians, historically. I'm not sure Aedui could efficiently rival them, but any Celto-Ligurian hegemony (like Salies) could have an interesting regional influence (while threatening for Massalia).
If Rome is still there, tough, I'd excpect Aedui hegemony to blossom as it did IOTL, tough rivaled by Arviovistos' hegemony in lower Rhineland. The decline of druisism is pretty much a fact, and the existence of a monetary unification in central Gaul would probably reinforce the client status of Aedui and "Ariovists". I'm not seeing Arverni to be that powerful, even if still a main player in the region.