IOTL, the Averni nearly conquered Masallia
You must be mistaken, Arverni mostly managed to establish their hegemony west of Rhone and they never really managed to curb down Volcae-dominated coasts (that were themselves took on peoples as Elysices) : aren't you confusing with Salyes and other Celto-Ligurian peoples?
Might we see a Hellenized Gallic nation arise and expand north
IInd century Gaul, at least for what matter Mediterranean and Central Gaul, were more or less importantly hellenized. It's particularily obvious in sites as
Entremont (one of the capital of Salyes) in Provence, or
Ensérune in Languedoc, but the hellenic influence probably went deeper, being probably one of the sources of the development of druidism IOTL.
Assuming Rome gets defeated in the IInd Punic War and remains, for now, an Italian secondary power (any earlier PoD more or less let us with a much more difficult task for describing changes in Gaul or Britain); and mostly depicting immediate changes for the IInd and early Ist century.
Arverni are likely to be unchallenged in the IInd century,
with their archê and sphere of influence spawning between the Rhone and to Limousin as IOTL before Roman campaigns.
I doubt, however, that this hegemony would be really stable, at least in a first time, and would probably focus on keeping clients and allied peoples close enough, thanks to the wealth of Arverni lands. At best, I could see Arverni taking victoriously on Volcae, but I'd rather see an interventionist policy there, rather than clientelisation. Arverni seems to have focused on the coastline mostly in opposition to any contender there, rather than out of particular interest.
Still, Arverni are going to remain one of the most powerful peoples in Celtica.
Allobroges are likely, IMO, to form another regional power ITTL. They seems to have formed an archê of their own in Alpine and Rhodanian Gaul (
the roman map of the region is a good hint, if probably reducing), up to Provence, and part IOTL of the anti-Roman coalition led by Arverni.
I'm not sure Aedui could efficiently rival these hegemonies, and likely wouldn't come to the same degree of dominance they did IOTL, thanks to Arverni and Allobroges power being curbed down and thanks to their alliance with Rome. Still their important strategical and commercial localisation would help them to remain independent and to form a secondary power of their own, with a whole network on the Seine basin (
mostly as IOTL)
Now, let's remember that these hegemonies are still pretty much decentralized, and more along the model of complex chiefdoms at this point : the independence of tribal states, even if clientelized/vassalized/allied is to be taken in account.
Mediterranean Celtica is a bit of a problem, especially in the western-part : it's more or less assumed that you had some big troubles in the region in the wake of Punic Wars (Enserune being fortified,
Pech Maho (Saiganthé?) collapsing, etc.) but it's not really clear who was involved and how deeply : it's possible that the destructions are partially due to Volcae, allies of Carthage, when taking over the region.
As for the eastern part, namely with the Salyes and other Celto-Ligurians (especially Cavares), I doubt they would be that able to take on Massalia even if they wanted so. Conversly, Massalia prooved unable to really take on their neighbours (that she used as auxiliaries from time to time), as hinted by Thelinè/Arleate falling outside their control.
It's to be noted, tough, that without Roman support, the Massaliote pressure on its hinterland would be significantly diminished.
The distinction between Celto-Ligurians and other Gallic peoples shouldn't be overestimated : the principal difference between, say Volcae and
Cavares is a matter of local policies and influences (such as hellenization)
How these entities would evolve?
Well, in a first time, we could wonder if druidism would encounter the same decline it did IOTL in the IInd century BCE. I'd rather think not, at least not in the same proportions : a good part of late Independent Gaul history is dominated by the Roman influence : institutionally (rise of vergobrets), economically (gallic coins whom value based on denarii), culturally, etc.
That said, without Rome, you wouldn't have a vaacum and there's an open question about who would take the lead in Mediterranean. With the PoD I choose, the answer is obvious : Carthage.
Or rather, parts of Carthage, especially the most "imperialist" factions such as led by Barcids. I wouldn't expexct at the latest an Carthagian takeover of southern Gaul (would it be only because Barcid rise was unstable and IMO, had good chances to met an unexpected and sudden end), but the mercenarisation of peoples as Volcae by Carthagians would likely be a good replacement for the same militarisation due to Rome (albeit in a different manner) in southern Gaul.
It seems it was already happening tough, with the Celto-Ligurian
dunastai, for instance, and the militarisation of the Salyes-led confederation.
It's possible that the attested demographic pressure of the late IIIrd/early IInd (partially due to natural growth, partially due to hinterland pressure) might have influed there as well, as much as foreign presence : the social need for safety, but as well a growing stratification and specialization of urban and peri-urban ensemble, both allowed and forced institutional changes (part of the reason for the militarisation of institutions in late independent Gaul might be the need to assert the social order).
It's still pretty much based, for what matter Celto-Ligurians, on a rough equivalence of power between various Celto-Ligyes entities, hence why Salyes-led, rather than Salyian confederation.
As for hinterland hegemonies, you have a similar situation, enough room for the development of strong cyclical chiefdoms as Arverni, altough I'd expect less political equalitarism and more of a roughly unified decentralized network (as between the Arverni oppida of
Corent, Gergovia and Gondole, possibly forming sort of a meta-capital due to their closeness; or with the contracting yet growing oppidum of Bibractos for Aedui).
I wouldn't hold my breath for a punic influence akin to Roman one (most of Carthagian features north were essentially hellenized), but an institutional influence, making vergobrets turning more sophet-like rather than senatorial-like? That's a possibility, altough such influence would be really thinner than Rome.
Now, there's the special case of Cisalpine Gaul : Rome not being destroyed in all likeness, you'd see peoples allies with Carthagian during the Punic Wars, but probably left to themselves (a bit like the really vague and probably still-born Italian federation opposed to Rome). I doubt that Rome would take back the region any sooner (being more focused on the mess in southern Italy for a time), but at the very least, it's going to be closer to IOTL in this region than in any other part.
In Britain, will some type of urban civilization still arise?
Britain would be likely fairly unconcerned by the changes in Mediterranean, as long as the broad structures aren't damaged. So, I'd expect hill-fort network to be one step towards an original urbanisation, as oppida were in Gaul.
Safe from that, I don't see any good reason why Belgae wouldn't get expanded in Britain as IOTL. So, what happens in Britain is essentially tied to what happens in Northern Gaul.
You notice, maybe, that I didn't mentioned too much Belgica or Armorica : well, it's because there's not much to be said there. Eventually, they would be fairly untouched by the southern changes, and you'd see the appearance of hegemonies built on trade as Veneti or Aremoricani; and hegemonies built on military dominance as in Belgica.
I'd expect a similar evolution in Britain, at best, as these regions would be rather on the "recieving end", IMO.
Might we see some conquerer unify some of the tribes in southern Britain?
Probably not : political structuration of Britain seems to have been (altough it's really hard to proove) begun later than in Gaul (with the principalties of the Vth).
What happens when the Germanic tribes expand westward? Might we see the Gauls flee to Italy and Hispania? Would the Germans still land in Britain?
The distinction between Gauls and Germans in any pre-Caesarian consideration is mostly moot. Most of Germania proper was either Celtized to an important extent, if not Celtic (especially for Danubian and Upper Rhinish Germanias) : the "threat of Germans going to invade Gaul" is mostly, if not only entierly, Caesarian propaganda that rely on depicting Gaul as civilized, and Germans as brutish.
In fact, you didn't have that of difference between both banks of the Rhine, including linguistically (Ariovist is a celtic name, for exemple). The mess of people in Belgica points how much the difference can be irrelevant culturally and politically.
Anyway, without the destabilisation of Gallic policies by Rome, you won't have as much room for opportunist expeditions as Teutoni and Cimbrii (which were joined in a really large manner by Celts, and probably at least Celtized themselves), and probably the same waves of migrating groups you had in Gaul since the Vth century (such as Volcae imposed themselves in Languedoc during the Punic Wars).
