Here are some national characteristics rules our group has been experimenting with.
Austria: We feel the battalion mass rule, large units, and optional irregular grenzer rules give the Austrians appropriate national characteristics so we have added nothing.
Britain (and British trained Portuguese): 2 rank line: Infantry in line receive one extra shooting dice (on top of any skirmisher advantage) but are penalized -1 melee dice.
France: Flexible tactics: French infantry rated experienced in all periods and theaters may move full after changing formation from attack column or line while French infantry rated amateurs in all periods and theaters may move half in the same circumstances. Enemy proximity reduces this to half for experienced and none for amateurs.
Prussia: Superbly drilled: Prussian infantry in the Conquest period may move half after changing formation even though they are linear tactics. Enemy proximity has no effect on this. In the Empire, Liberation, 100 Days Prussian infantry get the same Flexible tactics rule as the French above.
Russia: Stoic infantry: Russian infantry units are broken when they take +1 Disruption over the number of bases in a unit (usually 5 Disruption will be needed to break a Russian infantry unit).
Spain: None. We feel the existing list reflects the Spanish well with the Home Field Advantage and Unpredictable rules.