What would have happened if Rome had put more pressure on the western German and Celtic Tribes than they did on Greece? Would the Roman way of thinking made German, Gaelic, Celtic, and Spaniard nations when it broke apart? Would it stay together?
Dude...
Okay, first off, Susano will kill you if he catches you using German instead of Germanic before... 945, I wanna say, but I forget.
Second off, Gaels are and were Celts. Third off, Spaniards were largely of a mixed Celtiberian culture. So there's a good bit of redundancy there.
Fourth, I'm not really sure how much more pressure would be plausible. Ireland and Caledonia were worthless from a Roman point of view. Entirely unsuitable for Mediterranean agriculture, no booty worth subduing every single tribe, no enslavable population to speak of. Most of those also apply to Germania, but with more people, and people after them and so on...
The attempts OTL, pressing Brittania's border northward and the disaster with Varus, well, they just weren't seen as rewarding enough to be worth the cost.