France needs to kick some ass to become a superpower with a POD no earlier than 1900. Germany is the country which needs to have its ass kicked.
So...a few things others have mentioned which will help France wipe the floor with Germany:
1. An increased birthrate would definitely help. Lets go with something as (seemingly) unimportant as a 1% growth per year, due to a rather enlightened government realizing that France needs more manpower. Eventually, good proportions of this extra population will go forth to Gallicize the colonies.
2. World War I starts under slightly different conditions, preferably with everyone except France involved. Socialist French governments before the 1920s help with this, since they were relatively friendly towards Germany. Perhaps France has effectively by 1920 become nonaligned with the power blocs of Europe, kind of like that one superpower...what was it called...the USA?
3. World War I goes on for a
long time before the French get involved. Five years should do nicely. Its even better if America has been involved from the very beginning...especially against Britain. UK-Germany-Austria vs. US-Russia-Italy. Russia should have been strengthened earlier on in the 20th century with a liberalization following a 1905 revolution, so as to be able to seriously hurt Germany and A-H. Getting the US to go against the UK is the difficult part.
4. When the French do get involved, the war ends within a year. The addition of a huge French submarine arm (socialists' idea of cutting back naval armaments by funding lots of subs instead of battleships) to the Atlantic War already infested by large numbers of US subs brings an exhausted Britain to the table. French tanks (since great French generals have paid close attention to the war being fought, and put some of its lessons into practice) drive towards the Rhine, through Belgium and the Netherlands. Russia finally breaks the Germans and Austrians, and Italy starts gobbling up the Balkans with the help of latecomers Romania and Serbia.
5. The UK ends up being dealt with rather harshly, losing colonies to France and the USA. France gets all of Germany west of the Rhine, plus the Ruhr. Austria-Hungary collapses into three or four different countries, and a severely exhausted Russia expands a little bit to the west.
6.
USA- Over the next decade, the social and economic ravages of fighting a total war against Great Britain and winning after a struggle of six years sends America into a spiral of depression and political and social unrest. The colonies she took from Great Britain rebel and revolt so often that America usually ends up granting them independence.
UK- In the 1920s the UK goes Fascist. But not in the militarily competent German sort of way, more in the militarily
incompetent Italian way.
Germany- Germany goes communist and isolationist, just perfect for France. Maybe this brand of communism is agrarian, and Germany self-imposes a Morgenthau plan on itself, without the starving civilians. Germany becomes a really big happy Finland-type state until the present day, with an internal war every decade or so to keep the economy suitably low.
Russia- Russia burnt any stability it may have had under the Tsars' reforms in a six year World War. The nation soon collapses into a civil war that will not end until the late 1930s, utterly devastating the nation.
France- Since the end of the war, France has added Belgium, the Netherlands, the Rhineland, the Ruhr, Alsace-Lorraine, all of the Dutch colonies, and several British colonies to her domains.
OK...now you have it intil 1930-ish, I see the superpowers being USA (though weaker than in OTL), and France (USSR-strength). A war in the 1940s which pits these two against a "losers' coalition" of the UK, Russia, Italy, Spain, and some Balkan states will allow France to gobble parts of Northern Italy and Catalonia, and impose an occupation over southern England

eek: ) and large parts of Eastern and Southern Europe.