The 3rd century is pretty early, but a more successful Arminius and Romans still pressing against Germania might get somewhere in a manner akin to what was going on in Gaul at the time of the Roman conquest there. But generally Germania was quite poor, too poor to encourage a real unification at that point. I don't think it would be anything that could survive a single ruler, or a single defensive war with the Romans since the incentive was too high for any individual tribal ruler to ally with Rome to deal with his Germanic enemies (see Arminius).
I used to be a fan of this scenario, but given the political and economic realities of unifying Germania in that era, I think it would have to be a project that starts in that era, but wouldn't finish until the Huns come in and make Germania the center of their empire (which TTL would have to survive the issues post-Attila of course). By Late Antiquity, Germania seems much more conducive to unification, hence why the Franks had such success from Clovis's era onward.
It wasn't so much a plain back then as it was a dense forest punctuated by marshes. Not quite good horse country.
Not sure why you need Huns to create an empire from the outside when you are give centuries to work with(especially if you go up to 400 CE with the ATL rather than try 200 CE which is admittedly pretty hard to work with).
after Teutoburg forest but unification must be complete by 3ard century and it must coexist with Roman empire of atleast 395 borders
During the time there wasn't any obvious way to create an unstable situation in the Roman lands of frontier, the best short term outcome would have been exhausting Tiberius' punitive expeditions to the point where they either retreat in disgrace, lessening Roman influence in the region and emboldening other people to join the Cherusci or remove Roman yoke, which while it does not necessarily cause major differences in the short term it makes the border more unstable and the Cherusci can find more friends in the longer to raid the frontier which can easily accelerate the process of formation of coalitions by decades if not generations.
What could be interesting is to have a proper Germanic coalition participating in major political events in Gaul like the ones we saw with the Batavian revolt or year of 4 emperors.
By 100 AD it could be that the Romans only partially conquer Britain(no Brigantes?) and that the Agri Decumates are also not conquered which would give more land for Germanic groups earlier on.
From there you can have the Cherusci-lead coalition slowly grow and consolidate, seeing the same processes we saw OTL but accelerated by means of more stable and less compromised leadership(with possibly favourable deals with Gallo-Roman administration depending on the aftermath of their intervention in the ATL political troubles)
Like
@TheCataphract said developing agriculture is rather important, the Cherusci's homeland was pretty close and in part contained some of the most fertile land in Germany so if they seize Thuringian, Eastfalia, Westfalia and upper Saxony they could demographically dominant the region enough to make neighbour tribes subservient to them in the context of conflicts against an aggressive Roman empire or even within the context of ambitious raids towards Gaul.
In this timeline a Marcomannic war could be triggered in the context of Cherusci pushing some tribes from the northern plains towards the frontier causing the Romans to decide to launch a campaign to pacify the region as Marcus Aurelius tried, here there is a strong faction looked deeper in Germania that can play a diplomatic game itself and try to ingratiate themselves with Germanic tribes near the frontier that are threatened by the Roman incursions.
If alongside this we see major unrest in Britannia(or British raids if it's not conquered) and also Dacian incursions into Thrace and Pannonia and even Jewish rebellions and Parthians incursions we could stack the deck against the Romans enough to see major defeats and retreats in the early 2nd century CE, possibly abandoning the upper Danube frontier and Pannonia. Cherusci allies, clients or simply Cherusci sub-groups could split off and take over these lands while retaining diplomatic and social ties to the core Cherusci homeland, moving into the year 200 we might not see a unified Germanic state or even a single Germanic coalition but we see a dominant core that feeds expansion of allies and splinter groups in a Roman empire that has less strategic cushions and is weaker a a couple generations earlier than OTL.
In the third century we could see through some parallel political and economic crisis period a weakening of the Rhine frontier(not necessarily through deep migrations as OTL), abandonment of Britain and Italy being open to deep raids as OTL.
Goths in this timeline could be one of the few groups that is far from Cherusci influence but could indirectly cooperate with them as their interests align, though the Goths would be mostly coastal raider as Dacia would probably survive at least that point.
In this timeline the Franks could be a more conscious creation of the Romans to counter-balance Cherusci(or whatever the name of a newly born macro-tribe is going to be, just Suebi or Alemans maybe?) which could be a double edge sword as similar processes of frontier collapse could happen as the Franks slowly become accepted as the authority in Toxandria and Belgium.
The Marcomanni would be the major thorn in the side, cooperating with the Romans when convenient to them but also taking advantage of the fact Romans can find themselves in unstable periods and raid over the Danube in Noricum and Bavaria.
Saxons or an alternative coastal macro-group would have amicable relations with the Cherusci/Suebi and be eyeing the Gallic and British coasts for raiding opportunities and not having major border conflicts with their southern neighbours.
Where OTL the Alemans were located there would be a Cherusci/Suebi derived group which would be just as bellicose and daring as OTL Alemans, raiding into Italy, Southern Gaul and so on.
The remaining parts would divided between Cherusci-aligned groups(mostly in peripheral lands neighbouring the Cherusci core) and Roman clients near the Rhine border(Chatti in the Rhine-Main confluence)
We could see by around 250 CE Germanic groups, some friendly to the Romans and some completely hostile, controlling both sides of the Rhine, all the Danube basin and croaching into Illyricum and beyond Belgium in Gaul, either the Parthian or some successor state would control Armenia and the Upper Euphrates basin and threaten Central Anatolia and the Levant, with a lot of unrest in the Gallic and Hispanian countryside.
As the more successful Roman emperor(s) slowly subdue the crisis we could see the Romans pinning down the large raiding parties and fight in a large pitch battle between a Roman allied side and a Suebi-Alemannic coalition in North-Eat Gaul on the scale of the OTL Battle of Strassburg in the mid 4th century or even some sort of alt-Catalaunian fields, with enough we can have the anti-Roman coalition rout the Roman alliance and permanently destroy the Rhine frontier and push the Romans south of the Alps(East of the Rhine).
In this timeline while raiding go deep and sack of large cities do happen, the Germanic tribes don't assume the same kind of military positions as they did OTL in the late 4th century CE or 5th century CE and there are no ATL Huns(yet anyway) so these 3rd century events amount to a more coherent territorial retreat from Toxandria, the general Rhine frontier and even Belgium.
In the Balkans the Romans hold unto Illyria but West Germanic groups routinely raid into Veneto and the Dalmatian coast, the Dacians are also a thorn to the side but the lower Danube frontier is more solid, the Greek cities in the Black sea are conquered by the Goths and the grain trade there is either interrupted or exploited by the new rulers to their own gain.
In Gaul the Roman frontier now sits on the Somme, Ardennes and Vosges mainly, with a defence in depth and by relocating loyal Franks, Chatti and other groups on these thinly populated border regions, the Suebi/Cherusci now directly or indirectly control virtually all of the Rhine basin, in the 4th century CE we will see incredibly ferocious fighting between Romans and hostile Germanic groups between the Loire and the Rhine as the Romans try to push the Germans back beyond the Rhine and the Germans use the weaker frontier as an opportunity to incessantly raid.
By 300 CE Germanic high kings have become commonplace and start to be codified beyond ad hoc coalitions and impermanent alliances, we also see increasing scale of fighting between formely allied or cordial groups within Germania as central authorities increase and try to permanently tie peripheral groups to themselves and to remove the threat of other local kings, as the Romans are now far away the Suebian tribal group ends up converging under a stable kingdom lead by a single hereditary high king stemming from a prestigious family(maybe with indirect ancestry from Arminius?) but with powerful sub-kings.
This powerful state would then shift its bellicosity outwards pushing for direct conquest of all Germanic lands up the Roman frontier in the West and South, the Saxons in the north which would in part migrated to Britain would be conquered, in the East there isn't particularly lucrative land but the border could be pushed up to the lower Oder, in the south the Marcomanni would be conquered.
By around 400-450 CE we would see a larger Germanic kingdom from the Oder/Pannonia to the Ardennes and from Holstein to the Alps. Beyond this time this state would have to deal with detoriarating climate and internal matters, its influence within the larger Germanic world is obviously immense and Germanic peoples in Scandinavia, Denmark, Britain, Poland, the Carpathians and Black Sea would look towards it for inspiration and prestige, we could see in the aftermath of the climatic disasters of the early 6th a kingdom which could itself repopulated Poland, Pannonia and other depopulated regions and maybe conquer the Goths and subjugate the British Saxons directly, but the timeline is already pretty long as it is.
It's not by 200 CE and it's not ALL Germanic people but I don't think anyone could create a convincing timeline where a single state from Norway to the Carpathians and Upper Rhine exists in just 2 centuries starting from Arminius.
Even then this state I envisioned could have something like 6-8 million people by 400-450 CE and be in a position create a permanently parallel cultural and political beacon in Europe in the centuries to come, especially if we see the Roman state permanently fragment or partially fall to enemy states.
