If this is a "independent Frankfurt while the rest of Germany has unified" challenge, specifically with the German unification, I'd suggest that the best way of doing this is for France to somehow keep small parts of the Rhineland from after the Napoleonic Wars. Then have that area change hands a few times over the next century, with Germany taking it in a conflict, maybe like the Franco-Prussian War, a bit before WW1 (while ensuring that there are very few ripple effects and changes in the rest of the timeline to maximise the chance of a WW1 still happening - which is fairly likely, all things considered, if the course of history keeps going along a similar path. If you throw in some resentment to the Germans on the part of the Frankfurters at some point - maybe the Frankfurters do economically better by controlling their part of the Rhineland and extorting it to France, or the Germans utterly waste their city conquering it once or twice, or something, then at the end of WW1 then there is room for it to be turned into an international city, right around the time when other cities like Rijeka were being made into the same, and with all weird analogues like the little Moresnet territory (look it up), Danzig, etc, to rival the Saar due to an inability to integrate the city and its hinterland into the Saar territory.
Just an idea. Probably not the best.