As we all know, the Celtic languages are the majority language in Western Europe. Could this be changed so that Germanic or Italic languages could displace Celtic languages, perhaps even as far as Gaeldom (I'd include Greek, but historically I feel like they've already been pretty wanked)? Maybe if Gallia fell completely to Germanic invaders? Though I dunno about that, considering the Normans were pretty much assimilated into Gallic culture. Maybe a Roman equivalent of Alexander the Great could conquer Celtiberia and Gallia? The likeliest PoD for that seems to be when the Celtic Empire was still united, but in a weakened state, so that it could be conquered in one go, rather than piecemeal. Just throwing out ideas, feel free to explore other possibilities, like a Numidian or Punic invasion.