Roman conquest of western Europe is definitely not inevitable In a reasonable POD. Not sure if the Gauls could resist with a PoD during the Gallic Wars or just before that (it's not so hard to let the Britons alone, but I doubt they could become a dominant power during ancient times), but if for example Rome was ruined or lost the Second Punic Wars, the Hauls would be an easy next power in the region, as the Carthaginians would hardly advance in the mainland.
Not sure about Illyrians, but I think they were hellenized to early on to really build a distinguishable "post-Illyrian culture" around Europe.