With a little change to the mid-19th century (Napoleon III having significant numbers of soldiers fighting elsewhere or simply waking up on a different side of bed that day ==> no Mainz threat ==> no South German, especially Bavarian, adherence to the North German Confederation, though some states such as Baden might anyway ==> Austro-Prussian rematch with France taking sides to facilitate the creation of a Europe of nation-states in accordance with Napoleon III's ideas while, also in accordance with Napoleon III's previously established practices, doing it for some territorial gains, not for free; plus France being able to buy Luxembourg in the rather different diplomatic situation than the one which prevented that from happening in OTL) you could end up with a France that ruled Luxembourg, the Palatinate and Mainz, which could probably be assimilated due to the smallness of their populations. A Revolutionary France which got to the Rhine but never conquered most of Europe—leaving most of the rest of Europe under regimes that not only are bitterly reactionary but would be likelier than OTL to remain so due to viewing revolutionary ideas as foreign and allied to the hated French threat—would stand a chance at assimilating the people of the Rhineland as OTL France assimilated the people of Alsace, through a more civic than ethnic nationalism that offered them the chance for greater rights and liberties than they had enjoyed before.
But even a French metropole with truly extreme borders—e.g. the First French Empire at its height (though not the manner of achieving those borders that was done in OTL, as by that point Bonaparte had already antagonised enough powerful nations to make his defeat close to inevitable; slower expansion, probably starting off with Kings of France winning some of the wars that they lost in OTL earlier on, would be better)—could quite possibly make its lands ethnically French. Not only is there the OTL evidence (contrary to optimistic suppositions) that strict, no-compromise policies at enforcing national identity and opposing regional identity can be more effective than allowing concessions and limited change in the hope of averting independence, a state can take even more extreme steps than that. Once upon a time people might have thought it unthinkable that East Prussia could cease to be German; it could be taken from Germany but never made non-German. Such people would have been proven wrong. If a state dominated by one ethnicity is sufficiently worried about a group of people within its borders of a different ethnicity and the nation-state of that different ethnicity is presently too weak to object, it can simply expel that group of people into that other state and forcibly resettle the relevant piece of land with people of its own favoured ethnicity. Sure, a democratic government would have difficulty forcing its people to move away from their homes to resettle some empty lands, but not all governments are democratic; any number of possible governments—a revolutionary leftist dictatorship, an authoritarian monarchy be it legitimist or Bonapartist, a right-wing republican dictatorship, a nationalist semi-democracy where only parties that the state considers sufficiently moderate are willing to run, et cetera—could do such a thing provided that they cannot be quickly and easily deposed by popular will. Poland is an especially dramatic shape of essentially shifting a nation's borders by a large proportion of the breadth of that nation. No-one who knows what was done across much of Europe during and after the Second World War, except perhaps someone extremely idealistic, can honestly conclude that such ethnic cleansing is not a viable possibility to facilitate long-lasting border shifts.