I like the idea of a maginot line but it should be giving more specific bonusses, I think - lets see what history teaches us about it: it was not everywhere but in a specific region, eastern france along the rhine, so lets say if city of Straßbourg builds the maginot line all 8 tiles directly surrounding it should get get a movement malus (or even prevent it like fort does) for outside troops (- or much like the great wall gives in civ5), + enemy units on all the surrounding cities tiles should get fairly damaged, maybe 20% per turn (possibly could use the vulcano outburst code for that that, which suddenly gives surrounding tiles +1

and damages close units). Also, all the bunkers of a civ could get +15% defense.
The +2 happiness bonus in all cities is ok with me (citizens in nation feel save).
This modified way, it's still possible to get in the back of maginot line with tanks and crush defense, much like the germans did, if you find a hole in the bunker system to sneak thru. A barricade everywhere does not represent this, to my taste.