I'd go for the French first. Why?

-First of all, who is going to buy these sets? Or better: what persons are your target group?
Diorama builders or wargamers?
Wargamers buy much much more boxes than the average diorama builder and they terribly need figures in marching poses. It's just not possible to build a wargaming army out of most 1:72 boxes. Just take the Waterloo 1815 Poles: great figures, no doubt..but only 4 (!!) marchings figs, which turns them quite useless.
A proper French marching-only box will make napoleonic wargaming in 1:72 possible in the first place.

-Second should be Prussians. Anyone who wants to build a Prussian Army for gaming purposes needs Landwehr, as they appear in every list. And NO ONE has ever done a new Landwehr set, the ancient Airfix set remains the only one. This is a unique chance!

-Austrians: Well..Italeri recently have reissued their old set; maybe a bit dated, but still a competitor for your set of Austrians. Why not producing those sets first, which will be the first of their kind?

-Concerning the Russians: Zvezda has already done a set of Russians with a proper figure of 14 marching poses and a sensible choice of command figures. I know they are grenadiers, but its fairly easy to convert them to Line oder Jägers. So you'd invest into a product for which there's already a strong competitor on the market, wheras for the Prussians or French there's no competition at all