One word repeated against and again and again:
Rome! Rome! Rome! Rome!
They already had most of Europe- just have them conquer Germany etc. and we're done!
Rome is probably the major reason why Europe evolved separate. After all, from the most removed point of view, Europe
did spend most of its history under one power: Catholicism. Christendom. That kind of thing. It was made up of many little political units but one religion unit. The break-down of that religious unit is basically the story of European history from Charlemagne onward. Hell, it's the story of European history from Theodosius onward.
The problem is Rome wasn't exactly a European civilization, at least in the sense that Germany is a European civilization. At the time of antiquity, Europe was split between two different 'cultural spheres', the Mediterranean and the North European Plain. The Mediterranean was dominated by Greeks and Pheonicians at first but then Rome later. The North European Plain, on the other hand, was
Celtic, almost to a T (while Germanics and Slavs still lived in the area, the dominant civilization was Celtic). As long as the center of imperial power lay outside the dominant cultural sphere, Europe could never stay united in the long term.
Key to united Europe kind of like China would be preventing the snuffing of Gallic civilization in its cradle by Caesar. Gallic civilization was better adapted to surviving on the North European Plain and even thriving. One cliche in 'surviving Rome' TLs is the invention of the heavy moldboard plow to aid in farming the touch Northern European soil. The problem is:
Why would the Romans do this?. The majority of the wealthy and economic activity in the Roman Empire was in the Mediterranean, and their contemporary farming package was finely suited for Mediterranean agriculture. What little farming was done on the North European Plain, in its tough soil, was large scale plantation agriculture, something not exactly conducive to technical innovation.
The Celtic Gauls, on the other hand,
lived in the area. The focus of a hypothetical Gallic Empire would be on expansion eastwards across the Plain, instead of eastwards towards Persia. In all honesty I've always been of the opinion that in no
realistic timeline could the Roman Empire be brought to the Dniester/Vistula border people seem obsessed with, and thus it could never survive in the long term because of the threatening external populations living in modern day north Germany and Poland. A Celtic Empire based out of Gaul, on the other hand,
could achieve such a border and survive in the long term.
And, of course, when gunpowder is finally invented, at some point in the timeline, it'll be descendants of Celts and Celticized Slavs rapidly moving east across Russia, the Urals, and Siberia, instead of Russians.
Imagine that, a polity that stretches from the British Isles to the Pacific. Scary.