I could come up with several POD, such a Gaulish victory at Alesia followed by Rome being to busy w/civil disorder attempt a large scale conquest, Carthage defeating Rome and not having much direct influence above the Pyranees or the Po.
But I'm not so concerned about the POD but what would the European cultural sphere look like? What would Celtic and Germanic culture would evolve into where the the classical worlds influence is not that of conquerers but as trading partners. Where Druids remain a going concern for centuries more and the first road builders are the Gauls and the Brits as they absorbed the tech of Greece and Rome but as independent powers.
What gods would they worship 1000 years on, or 2000? How divided would they remain. Would something like the Tain or the Eddas be the basis of "Classical Studies" and perhaps part of a "religous instruction". Would councils like the Allting and the Witangots be how kings were elected.
Basically, I'm looking at a world kind of like where the north won their Salamis and Platea. My question is, In your opinion what would that world look like as a seperate cultural sphere from the Classical world?