Celtic Republic 500 BC

[perfectgeneral]

What prevents the forming of a culture among Celtic speakers to rival or exceed that of the later Roman Republic?

As much a question about the starting circumstances of the Roman Kingdom and what the Celts lacked in all their lands at this time.

The Roman Republic (Latin: Res publica Romana; Classical Latin: [ˈreːs ˈpuːb.lɪ.ka roːˈmaː.na]) was the period of ancient Roman civilization beginning with the overthrow of the Roman Kingdom, traditionally dated to 509 BC, and ending in 27 BC with the establishment of the Roman Empire. It was during this period that Rome's control expanded from the city's immediate surroundings to hegemony over the entire Mediterranean world.

I would guess that a strong leader would be needed to establish a city. Perhaps a job for some ASB SI, but genetics could throw up an inspiring chap with a high regard for building. The language map shows the catchment area for a potential city/capital. Consolidating power in what is now the British Isles, France and Iberia suggests that the Parisii tribe and Lutetia might be a good place to start? Developing with a walled hill on the south bank.

The oppidum of the Gallic Parisii was on the Ile de la Cité, which at that time was smaller than it is today and was linked to the riverbanks by two bridges. It appears to have been occupied by the Parisii ca. 250-225 B.C.[3]
Lutetia became the chief city of the Gallic civitas Parisioruin in Lugdunensis Quarta, becoming Parisius in the 5th c. A.D. Roman Lutetia was founded above the flood-prone point where the Bièvre, a tributary of the Seine, flows into that river. It was centered on the slopes of the hill later dedicated to Saint Genevieve, on the left bank of the Seine, (which became known as the Latin Quarter in the Middle Ages when the Sorbonne University was founded.) Outlying suburbs on an island across from the confluence, the Île de la Cité, which became the Merovingian and modern centre of Paris.
In 52 BC, a year or so before the end of the Gallic Wars, the Parisi destroyed the bridges to the Ile de la Cité. However the garrison led by Vercingetorix's lieutenant Camulogenus, whose army camped on the Mons Lutetius (where the Panthéon is now situated), fell to the Roman military forces led by Titus Labienus, one of Julius Caesar's lieutenants. The Romans crushed the Gauls at nearby Melun and took control of Lutetia.

So this would be the outcome that I would expect to change by earlier sophistication of settlement here. A sentiment of valuing technology from Greece and the Mediterranean would probably be required.
The Grand Island of Strasbourg is another possibility. Argentorate. Better for Iron and coal? Bridging together Munich and Paris? Trade links to the Danube, Elbe and Seine? Are they are going to need a stirrup or something as an initial military edge? How lucky do you need to be to found an empire?