Interesting idea, but seems unlikely for several reasons...
1. The cost...the French are already in the middle of a costly war in Algeria+decolonization;
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No. Actually, the creation of what we call the "New Cities" (five great urban centers around Paris) has been decided, implemeted, and funded from 1961 to 1966. The Algerian War has not been any obstacle to that because the programm has begun after the war.
But I agree with you, a new capital is unlikely. We actually had a Brasilia of some sort, that we call Versailles, and the experience was not that conclusive. Paris as our capital city is far too rooted in French collective Psyche to lose its status for technocratic reasons. The least of the "cons" against such a plan wouldn't be the fact that historically, setting a Government outside Paris rings an alarm bell (usually, it means that Fritz or Reginald feels like invading us again.).
Actually, there would be very few cities of places apart Paris that could candidate for a capital status, given their localisation : Probably Orleans, maybe Rouen (which would have become a London in reverse, with the Seine playing the role of the Thames), and that's all. All the other major cities are too close to the borders and you can't have any decent capital in the Massif Central because of the cost of transport infrastructures. La Roche-Guyon would only have delayed the overconcentration of powers in Paris : it is now a suburb of the capital.
As for the architecture, you are all right here : it would have probably looked similar to Le Havre (although Le Havre, while not being my favorite destination for a honeymoon, is a surprisingly intersting city from an architectural POW, as well as a city-planning performance. I think it is now in the Unesco world Heritage list). This is what I personnaly call "Communogaullist" style : the grace and beauty of Soviet constructions mixed with the obsessive planification of the 60's style Gaullism.
As a conclusion, I also immediately made a connection with Blake and Mortimer. Did anyone here had nightmares as I did after reading the part with "La Chose" ?