1. French. Definitively. It's the main army of the period, just like WWII Germans, you simply can't do without them. Whether mid-empire or late, it's ok, just finish at least ONE of the two. My preference is mid-empire (fur caps, lots of cords, plumes etc), but I understand many wouyld love the late stuff, which is accurate for Leipzig or the 100 Days.
2. Keep the Austrians separate. The reason, below, under #3.
3. Those who collect 1812+ Russians, combine them with the late Prussians (already released by HaT). During the 1813 Spring campaign, the Austrians were not involved, it was clearly a russo-prussian affair. And while the Austrians did participate in the autumn fighting, one can do without. After all, the pro-Austrian collectors love the 1809 campaign.
2. Russian Infantry-Prussian Landwehr
3. Austrians with helmet and/or shakos, to cover the whole 1809-1815 period (and why not the 1821 and the even latter tappering shako to cover the period up to 1848???)