- Let's go with the POD that the heavy plough is invented as in OTL in the first century AD, but not in Han China but in Roman Gaul by a clever fellow who wants to help the heavier soils up north become more productive.
- This turns out to be quite the success; not only Gaul, but Britain as well starts to see a sharp increase in population. Slowly, the same starts to happen in areas bordering the Roman Empire.
- The increase in population leads to land clearances, the establishment and enlargement of many settlements, an increase in trade which accompanies the increase in number of consumers and arguably a cultural flowering under Roman auspices.
- Meanwhile, one of those areas bordering Rome is Bohemia, home of the Celtic Boii, who also begin to fall under the Roman spell. When the Quadi and Marcomanni come knocking, they enlist Roman help and manage to defeat them. Bohemia gets clientelised and given that Bohemia is mostly surrounded by mountains, the Boii (and Volcae and perhaps other Celtic tribes too) have the time to develop their polity. Let's say at some point they spread northwards toward Silesia and establish a presence on the southern Vistula.
- And would you look at that, with the Amber road becoming more well traveled and lucrative, with the proliferation of the heavy plough northwards, the Goths decide to stay put, leading to OTL Poland to become Gothland.
- Other areas nearby inhabited by Celtic tribes, such as the Agri Decumates, parts of Swabia and Bavaria and Slovakia undergo the same increase in population and development. While the plow spreads further northwards to the Germanic tribes, the Celtic tribes have the advantage of being both earlier adopters and already having a more sophisticated, more sedentary civilisation due to being more plugged into the Mediterranean trade network. The Celts spread, like IOTL, to Pannonia, Dacia and Moldavia, but perhaps this time they do not end up assimilated; I can imagine the Anartoi in the Carpathians and Bastarnae in Moldavia managing to reach critical mass before someone invades their lands, while Pannonia is always a crapshoot.
- This increase in food security causes more Germanic people to stay put and those that do migrate towards the Roman Empire are more easily rebuffed and even if they do cross the Rhine, they tend to be assimilated relatively simply.
- While Gallo-Roman and Romano-British culture come into existance and Roman influence is strong, the large increase in native Celtic population is enough to keep Gaul (save for Narbonensis and Aquitania probably) and Britain Celtophone regions.
- Perhaps the Crisis of the Third Century or something similar happens and the subsequent twofold split becomes a threefold one, between a Latin, a Greek and a Celtic Roman Empire. Perhaps the greater voice of Gaul within the Empire leads to more import being placed on the defence of the Rhine. Perhaps said interests lead to someone pulling an Augustus, a Charlemagne and conquering the by now sedentary Germanic polities. In any case, due to the fact that Gaul and Britain are plugged into the Noth Sea-Baltic trade network (their greater development will likely in turn push this network to develop a lot quicker than IOTL), it is possible at some point they go at it alone, because Mediterranean issues are not necessarily theirs. Gesoriacum bcomes the Constantinople of the north. Bohemia, Slovakia, Wallachia, Moldavia and Germany below the White Sausage line develop into Celtic polities as well, everything to the north of that is Germanic, Pannonia is Sarmatian or Alan because why not and Slavic Europe begins in Belarus and Ukraine.
- The heavy plough and increased trade it brought into existence have had distant knock on effects. It's the second half of the fifth century AD, Rome consists out of three distinctive parts. From the north, traders and raiders have started to frequent British and Gaulish shores. In the southwest, the Berbers are starting to raid more and more often, in the southeast, the Arabians have a surfeit of population and nowhere for them to go but to Persia and Rome, while in the east there are rumours of a vast horde heading west. Sounds like a good scenario for an AH version of Rome Total War.