Hello, and welcome on AH.com!
Well, it depends what would count as a victory, really : French army was badly unprepared to the war and the Second Empire didn't had much diplomatic support after falling right into Bismarck's plan and declaring war. It doesn't mean that you couldn't count on a strategical stalemate or even a Republican army managing to recover the situation after the fall of the Empire in 1871 (Guerre & Histoire made a fine allohistory about republican operations being successful into forcing Prussian to abandon the siege of Paris).
In these conditions, we'd roughly be talking return to status and probably some concessions being made by France (such as limited abandon of eastern fortifications, and non-ingerence into German affairs) while German unification is kinda bent to happen. If it's an imperial "victory" it would still count as a political decredibilization, far from the jingoistic mindset that launched the war.
Now, if we're talking on an actual strategic victory for the Second Empire, it's going to be much harder. But while relatively implausible, let's assume it's met.
The first thing would be that London, and probably Bruxelles, would definitely frown at a large scale French occupation of Rhineland and would force a negotiated peace as soon as possible. Not out of threat (at least for Britain), but out of focus on European balance. They wouldn't really be opposed to some French gains however, but it's probably going to be limited so except a particularily crushing victory, you won't see a dismentelement of the NGC (maybe, at best, reestablishment of some German states)
Interestingly, for all the talks about Germany, Prussia wasn't seen this much as an existential threat for France at this point, and a French victory would probably not that bent on curbing down unification as long as Germany remains a second-rate diplomatical power in Europe.
So, what was the French outlook on its N-E borders? Roughly this.
Black : French border in 1866
Dark blue : French borders in 1814 (First treaty of Paris)
Blue : Luxembourg
Light blue : Regions french état-major and/or diplomatic corps presented interest controlling or annexing
It's highly dubious that even in the case of a French victory, they could take all of the southern Rhineland (to say nothing, of course of Belgium). But Napoleonian goverment could ask for the return of northern Alsace annexed by Prussia in 1815 as well as Sarre. Nothing really valuable, but symbolic enough. Depending on the situation, maybe some places in Palatinate as asked for in 1860's as compensation from NGF, but I doubt it in these conditions (again, a total victory seems really implausible). Luxembourg is a special case : Nappie could argue that his offer of buying it could be remade then, giving that the whole Luxemburg Crisis was about Bismarck's opposition, but it would depend a lot of other powers' consideration. It could, arguably, be exchanged with Belgium in exchange for old 1814 places, but...