First, which kind of victory? Napoleon III pulling a Bismarck in Germany seems relatively hard, giving the relatively lesser (while not crushingly inferior) situation of french army logistics and operational capacities. We might end up with a French victory which would be not that much distinct from a de facto stalemate, which would probably make negociations managed trough international (reading British and Austrian, in all likeness) intervention.
Britain would pull a veto on any french troops west of the Rhine, not mentionijg any kind of french client-state, before someone could even end to say "natural bor...."
Anything more than return to 1814 borders and MAYBE Luxembourg and/or an limited extended border in southern Palatinate is definitely out of question for what matters London, which wasn't yet seeing Prussia as a potential threat for themselves. And that would be a maximalist annexation, probably much more reduced ITTL.
Belgium is definitely out of question, safe maybe for a compensation to France not annexing Luxembourg with a return to 1814 borders.
It's worth noting that French état-major or diplomatical corps didn't saw Prussia as an existential threat for France, but would have been fairly content with discrediting it as an international actor, rather than destroying it for shit and giggles.
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
I could see, however, London accepting partial dismembrement of the confederation on the North-West at the benefit of some princes defeated in 1866, but nothing too much benefiting for France.
As for the immediate changes, you'd may see something for German politics and maybe a structural and institutional liberalization of the German Confederation, no (at least in short term) unification with southern German states. But economically and geopolitically, Germany is still going to be a major player in the late XIXth.
In the same time, you'd probably not see the Second Empire falling anytime soon, while the liberalisation tendence it underwent began to weaken it a bit already, Republicanism remaining a popular radical alternative in most urban and industrial centers.
The absence of the leagues and communalist movement in France would have an interesting macro-historical consequences, as no socialist (or rather, semi-socialist) experience of power which would mark international revolutionary movement (anarchist or socialist).
Internationally, you might end up with a less polarized situation : as mentioned above, you didn't have at this point a real French antagonism against Prussia/Germany and while I don't see them being best buddies after the war, you would quickly end up with normalized relations earlier than IOTL (historically the normalisation of relations happened in the late 1890's)