First it is purely hypothetical because it will not happen. Suppose France is extra serious about "decentralization" and decides to move its political capital somewhere else from the disproportionally large ile-de-France, where do you think it should be located?
It would still be a sizable city because it will be too expensive to build everything from scratch. Second, it should be more to the south and farther way from Germany. And third it should ideally located in a region that is not the most developed.
So what about Clermont-Ferrand? A relatively underveloped area in the middle of the country, about equal distance between Paris and the Mediterranean?