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.)
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.)