A full blown union between France and Germany is borderline ASB with a point of divergence during the late 20th century. While there have been semi serious proposals made by politicians including Kohl himself, it would be frown with huge issues and would more importantly be massively rejected by the electorates in France and Germany.
The last thing Germany and Germans would want after paying for their own reunification would be to pay the sheenanigans of French farmers, civil servants and so on.
The European project has never really been massively popular in France outside of the elites, especially the political side of things. You can also definitely expect history and France and Germany past to come in and take part in any debate on a potential Franco-German union. I suspect the idea would fail badly in a referendum, outside perhaps of borders areas.
Organising a Franco-German Union would be no easy task too. Extending the Lander system to the French regions, is not an option as it would require massive transfers of powers to be done in a very short period of time. A significant amount of pre-existing French institutions would have to be maintained with clear power prerogatives, so as to make the Union accepted by the wider population. .
The social security systems would have to be completely merged as well, something which would in itself create huge issues due to their differences.
While it is not impossible to do so, it would be extremely difficult especially as said Franco-German Union would not make any sense at all on a strategic basis. Germany interests lie in central Europe as it is her own natural area of expansion. While France interests are divided between Europe and the remainder of this great big world. This basic fact means that France would not gain anything at all from a Union with Germany. Some parts of France like Alsace would gain big, but the entire Atlantic facade of France would lose big as it would become a fringe territory, likely ignored by the rest. Don't forgert that there are 60 million Frenchmen and 80 million Germans, this basic fact alone means that Germany would dominate said Union.