Well, it had been open three years prior...
https://en.wikipedia.org/wiki/Luxembourg_Crisis
Though, if Spain isen't in the cards I will admit Prussia is, geographically at least, going to start running into some issues finding a spot to prod at France. Maybe doubling down on their support/alliance with Italy and pushing for the return of Nice and Savoy (Perhaps throwing in Corsica), with Italy making some kind of legal arguement that the former two were seceded to Napoleon III's Empire? That's about the only other spot I can think of that isen't stepping on Belgium (Which would be a no-no for the British) and dosen't involve Prussia trying to project power colonially.
Well, instead of Napoleon III, Bismarck would have to converse with Theirs during the Seven Weeks War if the two aren't at war. As for Italy, Theirs could open more favor with them by pulling out of Rome though he may consider intervenining on behalf of Austria (or rather, just does not want Prussia to go sronger.)
Though maybe if France does help out Austria against Germany and everything goes smoothly, because France coulc convince Austria to give Italy some territory (granted, alot could happen to make things topsy-turvy. Maybe the Magyar nobles go for more radical attempts that end up de-stablizing the empire and has Austria focus more on German ambitions.)
Though if the Seven Years War goes as continued and France just wants to keep peace. I don't think Italy would ever get Corsica though. Maybe Nice and Savoy if France thinks it would get Italy on their side while also working to get Prussia from doing anything.
For France, maybe their ideal solution would be what I dubbed the "Three Germany" ideas. South Germany (made of the South German states and Austria), North Germany (made of modern-day northern Germany maybe led by Westphalia or Hanover), and East Germany (the Prussian lands and they focus on getting the Baltic states in some weird Eastern German quasi-nationalism.) This is me being semi-serious though.