So let's go back to the beginning of the Bronze Age and make the Celtics a lot more organized and united and even more agressive, thus allowing them to better fight off the Romans and the Germanics.  That is half the plot of a future novel of mine.

The issue is, I have to restructure ALL of Europe for this.  France would no longer be called so, so what WOULD it be?

And so on and so forth.

Thanks!

To make a long story short, yes, you would have to do this. No, none of us has the time or ability to produce the 2000-page book that this question would require. Yes, that means you're going to have to spend a few months researching. Yes, all good novelists do this. No, essentially asking a forum to make the setting of your novel for you is not a good idea.

r u irish

That's not to say we can't answer your questions incrementally, though.

What would France be called? Both "France" and "Gaul" are Latin... although that's not to say you can't make it a loanword.

You could start with Matasovic's Dictionary of Proto-Celtic. You'd need to find your grammatical information elsewhere, however.