French Cuirassiers and Chasseurs D'Afrique are excellent choices, Prussian Cuirassiers and lancers are must haves as well, 6 of each should do it.
A gun and crew for the French and the Prussians are also needed.
I'd then do French Chasseurs and either Guard Infantry or Turcos in their distinctive light blue uniforms, then I'd do Prussian jaegers and Wurtemberg infantry, that would give you four different infantry types for both sides, two different cavalry types and a field piece with crew.
The range would be very well flushed out and depending on sales, you could come back to it and do things like French Mobile Infantry, Marines, Saxon infantry as well as Wurtemberg Jaegers, then do French Carabiniers and Chasseurs A cheval and Bavarian Cuirassiers and Prussian Dragoons.
There are loads of other choices too, but the question becomes how well would they sell; the bottom line is as the company grows and this range grows in popularity, I'd think additions would make sense, they would be THE go to company for Franco Prussian War figures....................