AHC: Regions South of the Weser "Celtic" instead of "Germanic" in Roman times

[Salvador79]

IOTL, Elbe-Germanic groups moved Southwards from the 6th, 5th or 4th century onwards, and a number of regions previously within the cultural sphere we linguistically call "Celtic" (or archeologically "Latene") were associated mostly with Germanic-speaking inhabitants in Roman times: the Rhineland, Hesse, the Neckar region, Bohemia...

Your challenge, should you choose to accept it, is to change the course of history so that, by the time Romans like Caesar, Varus, Tacitus etc. stoop their noses into these regions, what they encounter there is of a quality which causes them to clearly and unambiguously consider it as part of "Magna Celtia", not "Magna Germania". It is not necessary for them (or anyone else) to form the concept of "Germanii" at all, but groups speaking "Germanic languages" and sharing a successor of the Jastorf material culture should still be dwelling along the Elbe and the Baltic. There can be actual migrations of people from there into lands South of the Weser, but if that happens, then they must "Celticise" to an extent that above-mentioned Romans (or equivalent persons) would lump them in with the rest of the Celts.

(If you object to "Romans" coming a-conquering with a PoD potentially in the first half of the 1st millennium BCE, well, I don't care, the Mediterranean arrivals may well be someone else, don't focus on that if you can.)

 

[Fabius Maximus]

I think the main problem with the challenge is that we don't really know what exactly caused the Germanic expansion, so it's hard to say what POD would have been necessary to stop it.

 

[Salvador79]

I think the main problem with the challenge is that we don't really know what exactly caused the Germanic expansion, so it's hard to say what POD would have been necessary to stop it.

It is true that we don't know exactly what caused it.
That does make it hard - but not impossible to answer the question.
I see at least two possible solutions:
Either one chooses one (or more) of the most common explanations and tweaks it.
Or one changes Celtic history to such an extent that, whatever the OTL cause of the Germanic expansion, it is improbable to occur.

Some of the common explanations for the Germanic expansion are:
a) PULL FACTOR: the Celts themselves, this time as a strong force: i.e. the very attractiveness of their more hierarchically organised, half-urbanised, comparatively wealthier, skilled-specialist, wisdom-cultic civilization exerted a transformative influence on the social fabric of a previously poorer, less specialised, more acephalous clan-based rural society, so that Germanic tribes learned and restructured and become more forceful and went to take over the "land where milk and honey flow"
b) PUSH FACTOR: climatic changes caused them to move Southwards in search of greener pastures (that is an explanation I tend to hear with regards to the beginning of the Great Migration around the 2nd century CE, but I wouldn't exclude it for that earlier time frame, either)
c) PULL FACTOR: again the Celts themselves, but this time as a weak target: for whatever reason (Romans are high up on the list of possible reasons here, both directly and indirectly), the small and decentralised Celtic polities of the region lost their military strength and became easy targets for raiders (especially when combined with an aspect of a), i.e. their greater wealth)
d) PUSH FACTOR: similar to a) only that it isn't the Celts who caused it, but some other endemic reason which caused Germanic groups to centralise and militarise. (I know that is extremely vague but if you want, you can pick any plausible reason for that development and run with it, from technological developments over new trade routes to growing population numbers probably caused by technological developments, too).

I am aware that we don't even know the form of this expansion:
If these changes started out as the hiring of mercenary bands, or as whole tribes packing up and moving, or as economic immigration, or even as an imitation of Germanic culture by Celtic groups (although I find the latter somewhat improbable for the time frame I've given, it certainly occurred later on on a massive scale).
Like above, feel free to choose a plausible explanation, and modify it.

 

[Salvador79]

OK, I'll venture into this territory first, and you can shoot me off if you like. I'd love to hear of better alternatives, so this is just to start things off...

I'll go with a combination of explanations a) and c) - first a), then c): Celtic culture was very attractive and exerted important transformative influences from the 6th century onwards, but military power of the Rhenanian Celts was weakened especially in the 2nd century BCE by a mixture of reoriented trade flows and terms and arrested political development (both arguably with a heavy dose of "Rome" written all over it, although by far not as bad as things got in the 1st century BCE with the conquest of Gaul when Rhenanian Celts were completely isolated all of a sudden).

I'll call the cheap-and-easy change in Celtic history that might shield the Rhine-Fulda-Main realm's Celticity the "Norican solution": Just like Noricum was a confederation of allegedly 13 Celtic tribes who formed a kingdom (and then became friendly with Rome), I'll say a similar kingdom arises in the region. Allegedly, one of the larger Celtic tribes which lived there were called "Volcae" (many Volcae appeared in many parts of Celtica). Maybe they are the Volcae who IOTL under some "Brennus" (most likely a Celtic word designating a leadership function, not a proper name) then moved across the Balkans and into Anatolia, maybe not. Another known Celtic tribe roughly in the region would be the Treveri. I'll still choose the Volcae since arguably their name became a Germanic cognate for anyone who isn't Germanic-speaking (Welsh, Vlachi, Welsche, Wallis etc.), so there's a small hint in the direction that SOME Volcae lived in lands that bordered Germanic territories.

My proposition: One "Brennus" becomes the founder of a stable Celtic kingdom which the Romans will later name "Volcia". Like Noricum, it pursues a friendly policy vis-a-vis Rome after they've seen that Rome is not to be messed with.

I'm looking forward to your alternatives - and to your What-If thoughts on the above answer.