How about something a little more radical...
Operation Big Bad Europe:
La Tene culture fails to happen/spread. The Etruscans succeed in maintaining hegemony over Northern Italy/Rome, and spread their dominion to include all peoples related to Etruscans (Illyrian Veneti, Veneti of Italy, Veneti of Noricum, Rhaetians), and all of the Italic speakers of the peninsula. Athens attacks Sicily during the Peloponnesian War, loses, then the Etruscans conquer and pacify Magna Graecia. While digesting, the strength of the Getai and other tribes in the area of Thrace (due to the absence of Celtic intrusion) keep Macedon from unifying into a powerful state and it remains a collection of warring families and kingdoms.
The Etruscans colonise southern Gaul and conquer Massalia around the 370s BC, exploiting the resources and intensifying agricultural production. This causes a migration of Gauls (not Celts due to the lack of La Tene culture, but clearly somebody was living in Gaul beforehand) towards Germany. Iberians sense opportunity and move into modern Aquitaine, whilst themselves remaining without unification. Sensing an irritating presence and being a military superpower, the Etruscans take no chances and attack the Iberian tribes from land and sea. Aquitaine is taken from them, as is OTL Navarre, Asturias, and Catalonia. A bad time hits the Etruscans as a newly resurgent Achaemenid Empire defeats a pan-Hellenic expedition and reasserts control over Macedon, the Aegean, Ionia, and parts of northern Greece; at the same time, the Carthaginians decide that enough is enough and pursue aggression against the Etruscans around 280 BCE.
The plucky Etruscans win the long war with Carthage that follows, taking control of Punic Sicily, Sardinia and Corsica. Carthage instead turns to African expansion, and begins to colonise the coast of OTL Mauretania. Afterwards, the noble Etruscans rescue Greece from the new Achaemenid Empire, the price being Etruscan hegemony over the Hellenes. When Etruscan interests clash with Epirote ones, the Federation of Epirus is crushed, and Greece realises what a master it has acquired. The Greeks attempt to break free, lose, and the price is brutal. Athens, Sparta, Corinth and Thebes are all put to the torch. Etruscan garrisons become permanent. The final large Hellenic bastion of resistance to the Rasna, Syrakuse, is finally brought low in 175 BC.
The reliance of Etruscan on slaves and warfare has resulted in social movements that result in several long-lasting civil and philosophical conflicts due to the power of military commanders, which transforms the traditional oligarchical-kingship model of the Etruscan state into a true Imperial state. This also results in greater integration of the Alpine, Italian and Illyrian regions of the Empire. During this time the Empire expands to include all of Iberia, for its great mineral wealth. The final phase of the First Etruscan Empire sees its borders expand to cover all of Gaul in a series of bloody campaigns that end in 135 BC.
The aging Achaemenid Empire finally collapses, into the Indo-Persian state; the new regime spreads Buddhism across its domains, and the religion then spreads through Greece into the Etruscan realms. The religion is pernicious, and the Etruscan Empire begins to actively persecute it. But it is unable to do so, and eventually a Buddhist Emperor comes to the throne in 111 AD. Being a little unpalatable with traditional Rasna virtues, he heavily modifies Buddhist canon and theology into a form that results in greater social control; the Emperor is a single enlightened soul, willingly sacrificing his choice to remove himself from the cycle of reincarnation in order to allow the Etruscan citizens to achieve nirvana. However, the invasion of Gallo-Germanic tribes of 'Borgondione' culture was disastrous; Gaul was removed entirely, large parts of Illyria and the Alps were overrun, and for a time Greece was also threatened. For almost two centuries, the Etruscan Empire fractured, as Bordongione languages spread across Europe.
The rise of a new and strong dynasty signalled a change. The peoples of Dacia and Thrace were brought into the fold of the Etruscan Dharma as allies against the Borgondiones, the vast gold and silver reserves of the Empire were used to pay for large auxiliary armies. Illyria and the Alps were again made part of the Dharma, and soon the Danube was the new Imperial eastern frontiers. Soon, even Borgondione cultures were on the Rasna payroll. Those Germanic tribes considered conducive to the Dharma were spared, those not were brutally enslaved. The Etruscan domains now touched the Baltic sea. Of western Europe, now only Pritannike remained. The domains of Gaul became one of the most densely inhabited parts of the Empire, and with its renewed agricultural development able to feed a large population.
Haven taken this example thus far, you can see where it's going. Etruscan Buddhist culture glues its domains together whilst still allowing for local traditions, no other Empires are particularly interested in attacking it, and its huge military presence precludes much rebellion. After several dips, growths, slides, barbarian invasions, civil wars, by around 1000 AD or so it has taken over the whole of continental Europe aside from Scandanavia, Russia, and the Bosporus. Civil wars last, the Empire splits, but always the Guardian Emperor of the Rasna returns in a mortal form and re-unites the Dharma of Europe. Philosophy develops quicker than OTL, but technology and economic developments are regressed by the nature of the state and so Europe only achieves Industrial Era techology in the 20th-early 21st Century.
A little tongue in cheek, but I doubt that any suggestion for a real timeline uniting Europe will require any less blind luck. Part of why Europe is difficult to unite is dumb luck, like the pattern of migrations being responsible for certain population densities and cradles of civilization, but much of the rest is from practical factors; climate, geography, and relative feeding capacity of land before certain technologies are introduced. Any PoD that unites Europe needs to be old. I think my 600-500 BC PoD for this sample timeline is actually still too recent.